Hello, you didn't do some searching before attempting to flash a Weekly ROM version fom Stable ROM one:
No "Format Data" in TWRP = Get in trouble!
For your fastboot issue, are you using USB 3.0 port? You must use USB 2.0 port.
Hi Poney! Thanks for getting back to me. I am using the same USB port that I used when flashing stable ROM. How can I fix this? I have tried most variations of TWRP for Mi 10 (umi) - still stuck on fastboot!