Hi, thank you so much , I installed it « twrp-3.3.1-16-Mi9SE.img » and now it works, I have the pattern back on TWRP and I can install magisk.
I had some problem to push it on my computer , fastboot was rebooting each time, I have to try an another pc, install ADB moded by XDA : adb-setup-1.4.3.zip , mi suite, then do it again