You must remember that there are problems with Mi 10 (umi) and TWRP. You must have an earlier version
3.4.0 (LittleTurtle2333) in recovery memory, and leave it there. You can only boot (NOT flash) from Fastboot with 3.4.2b-1206. It takes just over a minute to load and may take a short time for you to be able to input your pattern (not password). If you remember that, you can use it quite freely. I have been using it successfully all the time, booted from Fastboot.
Going from stable or stock to weekly beta you always have to format data partition.
You should be familiar with the Fastboot process, and have available
TWRP 3.4.2b-1206, together with the ROM you want to install, on your PC.
You should only boot using this TWRP version through Fastboot - never flash.
Boot into this TWRP
Format data (confirm by entering 'yes')
Reboot back into recovery
Enable MTP (under Mount)
This will enable you to upload the new ROM onto your phone.
Install the new ROM,
Reboot to system.
Additional notes:
The command to boot into the TWRP should be:
fastboot boot recovery-TWRP-3.4.2B-1206-XIAOMI10-CN-wzsx150.img
There is often quite a delay before the TWRP starts, and sometimes a delay before you can enter your pattern.