@MrT10001 Thanks! Got it working, as highlighted below . One or two things where different than documented- Did you also had to replace "fastboot flashing unlock" with "Fastboot flashing unlock_critical" ? (My tablet is from spain from 2022..was yours newer possibly, if you did not had to modify this command ?)

Ah- and Sidenote on "bricking"/restoring device: Lenovo offers an good "Software fix" software on their webiste that can restore your Lenovo device. You can install that, connect the tab via USB, give it the serial of your device, and it downloads the latest firmware and resets everything..the only thing tricky- as usual- I believe you might need fastboot mode at some point- which is POWER + VOLUME DOWN buttons at the same (USB disconnected) and then the lenovo enters fastboot mode and is ready a) to get out of this endless loops and b) receive the firmware.
For me, it restored Android 11, the right one for UBports.

@floem7 said:

fastboot flashing unlock

I can may be help, just solved it for my Tab M10 X306F.

1.What system are you using to setup Ubport as host ? In my case it was Win10- that has a problem and requires some "MTK Driver" to talk reliably with tablets and phones via USB cable. Without, you can still get either adb or fastboot to talk to the device, but not reliably.

2.Before you can run Ubports, the docu recommends "fastboot flashing unlock"- replace that with "fastboot flashing unlock_critical" (or better: both, one after the other) and then it really gets unlocked. There seems to be no clear feedback when an unlocking is incomplete and also Ubport is not reporting this.

3.I installed UbuntuTouch 20.04..as some older forum entries shared 24.04 is not stable, but that might be no longer true- I drawback is, that I have quite a slow browser now (which seems to be better on 24.04). Anyway- just sharing - Point 2 is the critical obstacle.

I apologies- got it working!
I had two important obstacles, one related to the UBport:
1.Win10 somehow has a problem with talking to tablets and seems to require an extra driver, "MTK Driver"- I haven't verified this myself, but switched to Linux mint, where adb worked nicely and found the devices always. Anyway odd is that adb showed the device before I started Ubports, but later lost the connection somehow.

2.The TB-X306F could not get unlocked as described in the docu with "fastboot flashing unlock" but "Fastboot flashing unlock_critical". It seems, that command has changed somehow- my latest firmware before Ubports came was from 2024. No one talked about this parameter in the forums, but I found finally, that it seems to be an old syntax- may be Lenovo reused an old firmware, so that this came back. This changed command should be noted in the related documentation, not a problem of Ubport itself- it's just difficult to find, as there's no clear feedback that the unlocking failed partially, and Ubport itself only realizes it late in the process.

I have now Ubuntu 20.04 on it. Impressed, that I got it working, but it was not trivial.

After 10 Trials to install it, in all flavours..TB-X306F is not fully supported (may be some older do), and should not be mentioned as supported. I lost now 12hours on this topic and 150$ for a lenovo tab that will not work at all.
Thanks and best regards,
formel