A look back at the situation 
I tried to make a TWRP backup but I still had this error:
Can not create "/data/media/0/TWRP' folder (required key not available).
A possibility found in a tutorial was via TWRP to :
- Disable the PIN code that is supposed to encrypt the partition. Or avoid the security pattern in favour of a PIN. Bullshit. No result.
- Try to repair the partition. No result.
- Switch the partition to EXT2 and then back to EXT4. I finally manage to make a backup but the next step will tell me that it is not a stable backup.
I restart the laptop and I arrive on the MIUI installation screen… So the switch to EXT2 then EXT4 corrupted the data partition, I did a classic format. The rage 
I go back to flash the image then I reinstall the backup which is almost 3GB. And on reboot, I still have the MIUI installation screen… So the backup is corrupted. I’m in front of a formatted phone once again when I try to make a backup instead… 
It doesn’t matter, I only lost a few hours during which I had managed my applications (personal and professional profiles), there are more dramatic things in life. I’m struggling to find a Wi-Fi, I ask my neighbour to let me access his hotspot to activate my device.
And then my friend, @marmotte32 the one who had already helped me in the past and who had unfortunately shown me this tutorial for changing the type of partition, sends me another tutorial for making a backup. Except I have nothing left to save. And in another message, he tells me:
So you can start downgrading to MIUI 12.0, right?
Why not 
Since I’ve already formatted the phone, all I have to do is test the downgrade. This is the method I used. Because yes, I did it! 
- I install MIUI in light version, I unlock the developer access, I start the USB debugging then I try the downgrade manipulation https://www.yewtu.be/watch?v=5LIQHUYjFeQ to go from the recovery archive miui_RAPHAELEEAGlobal_V12. 5.1.0.RFKEUXM_dd92b7128a_11.0.zip to miui_RAPHAELEEAGlobal_V12.0.4.0.QFKEUXM_4087debc4c_10.0.zip
And the downgrade works!
-
Android 10 freshly installed, I go back to my neighbour to get his hotspot. I activate my device, it’s 10pm. And I say to myself, I might as well give it a try.
-
I follow the tutorial https://doc.e.foundation/devices/davinci/install that I’m starting to know by heart:
- ADB: system flash to install TWRP on the recovery mode (instead of the MIUI recovery tool)
- Reboot to recovery mode (otherwise MIUI reinstalls its proprietary recovery tool) [HOWTO] install /e/ on a Samsung device from a GNU/Linux PC successfully and, one adb sideload later with e-0.17-q-20210522-UNOFFICIAL-raphael.zip I can see the loading going on…
A few minutes later, the Grail is there: the device reboots and I discover with wonder a new wallpaper. I managed to install /e on my Xiaomi Mi 9T Pro. I can enjoy my free phone 
Well done to all those who make it possible to free our phones 
If you want to free your Xiaomi Mi 9T Pro, remember 3 things:
- To be able to install the current versions of /e, it is imperative to downgrade to Android 10
This is possible from the MIUI settings directly by having the developer option. We play with the system by following the video https://www.yewtu.be/watch?v=5LIQHUYjFeQ and it works.
- Error 7 in TWRP seems impossible to solve.
So you have to go for it and have previously backed up everything that can be (files, passwords, 2FA via Aegis, open source, which allows an import and export for example…)
- If you have any problems and need to get back to the default MIUI operating system, there is an easy and safe solution.
Download the fastboot file https://xiaomifirmwareupdater.com/miui/raphael/ corresponding to the version of your MIUI system previously installed (for my part, French version Mi 9T Pro EEA indicated by the reference MIUI V12.5.1.0.RFKEUXM) and follow this tutorial https://gist.github.com/j-jith/0c4950e8020d9bed0c3104608b07a49d
And that’s it!