@flohack
Thanks for your suggestion. I already tried it but it requires that I first enable developers mode which I can't do because of the bootloop.

@mrt10001
Thanks for your instructions. Unfortunately the problem persists.

I'm unable to install TRWT because it requires in addition to unlock, which I did, that I enable developers mode, that I can't because of the bootloop.

I tried also the XDA link but it didn't work. See the transcript below:
chris@christopher-System-Product-Name:~/hammerhead-m4b30z$ sudo fastboot flash bootloader hammerhead-recovery.img
target reported max download size of 1073741824 bytes
sending 'bootloader' (15358 KB)...
OKAY [ 0.710s]
writing 'bootloader'...
FAILED (remote: invalid bootloader image)
finished. total time: 0.890s
chris@christopher-System-Product-Name:~/hammerhead-m4b30z$ sudo fastboot flash bootloader transformer-recovery.img
target reported max download size of 1073741824 bytes
sending 'bootloader' (6180 KB)...
OKAY [ 0.420s]
writing 'bootloader'...
FAILED (remote: invalid bootloader image)
finished. total time: 0.600s

Do you have any clue on how to enable developers mode from my computer?

@rondarius
I tried also: sudo magic-device-tool.
It recognizes the device and then got the following execution:
Flashing legacy channel

Creating filesystem with parameters:
Size: 734003200
Block size: 4096
Blocks per group: 32768
Inodes per group: 7472
Inode size: 256
Journal blocks: 2800
Label:
Blocks: 179200
Block groups: 6
Reserved block group size: 47
Created filesystem with 11/44832 inodes and 5813/179200 blocks
target reported max download size of 1073741824 bytes
erasing 'cache'...
OKAY [ 0.200s]
sending 'cache' (13348 KB)...
OKAY [ 0.650s]
writing 'cache'...
FAILED (remote: flash write failure)
finished. total time: 1.031s
Creating filesystem with parameters:
Size: 29236371456
Block size: 4096
Blocks per group: 32768
Inodes per group: 8192
Inode size: 256
Journal blocks: 32768
Label:
Blocks: 7137786
Block groups: 218
Reserved block group size: 1024
Created filesystem with 11/1785856 inodes and 156120/7137786 blocks
target reported max download size of 1073741824 bytes
erasing 'userdata'...
FAILED (remote: failed to erase partition)
finished. total time: 0.162s
rebooting into bootloader...
OKAY [ 0.083s]
finished. total time: 0.133s

2021/09/01 18:41:37 Device is |hammerhead|
Cannot parse channel information for device on http://system-image.ubports.com/ubports-touch/15.04/devel/hammerhead/index.json

Move to your device to finish the setup.

Cleaning up..

Exiting magic-device-tool. Bye Bye

@rondarius

I ran the UBports installer:
Got this first mesage:Error: fastboot:flash: Error: Flashing failed: {"error":{"code":1},"stderr":"FAILED (remote: 'flash write failure')fastboot: error: Command failed"}
I clicked on "ignore" and got this message: Error: fastboot:format: Error: formatting failed: Error: {"error":{"code":1,"cmd":"fastboot format cache"},"stdout":"Creating filesystem with 179200 4k blocks and 44832 inodes\nFilesystem UUID: 1b2fac8d-da28-402d-911a-7799676ee8fe\nSuperblock backups stored on blocks: \n\t32768, 98304, 163840\n\nAllocating group tables: 0/6\b\b\b \b\b\bdone \nWriting inode tables: 0/6\b\b\b \b\b\bdone \nCreating journal (4096 blocks): done\nWriting superblocks and filesystem accounting information: 0/6\b\b\b \b\b\bdone","stderr":"mke2fs 1.45.4 (23-Sep-2019)\nSending 'cache' (76 KB) OKAY [ 0.220s]\nWriting 'cache' FAILED (remote: 'flash write failure')\nfastboot: error: Command failed"}
Ignored again and then the UBports Recovery window opens with several options. The installer believes Ubuntu Touch was installed but the phone returns to the google start window and stays in loop.

@rondarius
Thanks for your you instructions. Unfortunately it didn't work.
I ran the first line of the script as you can see bellow:

chris@christopher-System-Product-Name:~/hammerhead-m4b30z$ fastboot flash bootloader bootloader-hammerhead-hhz20h.img
target reported max download size of 1073741824 bytes
sending 'bootloader' (3124 KB)...
OKAY [ 0.320s]
writing 'bootloader'...
FAILED (remote: flash write failure)
finished. total time: 0.500s***

And then, the second line:

chris@christopher-System-Product-Name:~/hammerhead-m4b30z$ fastboot flash radio radio-hammerhead-m8974a-2.0.50.2.30.img
target reported max download size of 1073741824 bytes
sending 'radio' (45489 KB)...
OKAY [ 1.690s]
writing 'radio'...
FAILED (remote: flash write failure)
finished. total time: 1.870s

And finally, the third line:

chris@christopher-System-Product-Name:~/hammerhead-m4b30z$ fastboot -w update image-hammerhead-m4b30z.zip
extracting android-info.txt (0 MB)...
extracting boot.img (8 MB)...
target reported max download size of 1073741824 bytes
archive does not contain 'boot.sig'
archive does not contain 'dtbo.img'
archive does not contain 'dt.img'
extracting recovery.img (9 MB)...
archive does not contain 'recovery.sig'
extracting system.img (996 MB)...
archive does not contain 'system.sig'
archive does not contain 'vbmeta.img'
archive does not contain 'vendor.img'
wiping userdata...
Couldn't parse erase-block-size '0x'.
Couldn't parse logical-block-size '0x'.
mke2fs 1.45.5 (07-Jan-2020)
/tmp/TemporaryFile-bYaV2x: Função de biblioteca ext2 não implementada durante configuração de superbloco
/usr/lib/android-sdk/platform-tools/mke2fs failed with status 1
mke2fs failed: 1
error: Cannot generate image for userdata

I had the fastboot unlocked but it seems that it still requires that I enable the developers mode witch I can’t do because of the bootloop.
Any idea on how to enable the developers mode from my computer.

@rondarius

Indeed, a very good post by Rojtberg but still can't get rid of the bootloop.

Following the post I got up to the point where I could run "fastboot boot image/CF-Auto-Root-hammerhead-hammerhead-nexus5.img"

Doing so:

1. It detects the devices.
2. Fails to Mount System and Cache.
3. Starts Rooting with SuperSU.
4. Runs SuperSU installer
5. Runs Boot image patcher
6. And automatically reboots.
   And then returns to the old bootloop.
   It looks like the procedure was unable to install the SuperSU app because what is need is to, first, reinstall the whole system and not only install SuperSU.
   Is there a solution for this?
   Thanks.

@rondarius Thanks for your post. I have got to the point where I was able to unlock fastboot and even got to UBports recovery screen. From there on, it freezes. The link you provided looks like a very possible solution. I will report the result as soon as I have gone trough the indicated steps.

@applee Exactly that. I will try "Rondarius" suggested link and report if it worked.

@capaterson I should add that I ran UBports installer using Ubuntu 20.04 and after unlocking the device and connecting it with an USB cable and ignoring errors, I got to the UBports Recovery screen. It would all work if I knew how to enable developers mode from my computer since I cannot do it from the cell phone.

@lakotaubp Thank you for your reply. Unfortunately, it's not the power button. You can see it's working well because I have access to fastboot and recovery screens and have the device recognized using adb devices.