Ubuntu Touch is installed!
Yesterday, at the end of the afternoon, I wanted to restart the installation.
In a few words, here's what I did.
I downloaded the image of hammerhead for nexus 5 (v. 6.0.1 full OTA M4B30Z = 588 Mo).
Then I installed it with TWRP > ADB Sideload.
After that, I activated again: Developer Mode, USB Debugging, MTP File Transfer Mode.
With the hammerhead version, unlike the marshmallow version, the Nexus 5 was immediately detected by UBports Installer.
Now, everything works perfectly, Ubuntu Touch is fantastic, goodbye android.
Thank you very much rocket2nfinity for all you gave to me.
John Wolf from France