"fastboot set-active" command fails I'm trying to flash a custom ROM on my Xiaomi Mi A3. When I flashed the ROM using TWRP and rebooted my phone, it got stuck on fastboot mode, won't even boot to recovery.

I found a friend with a similar problem and he said that all he did was changing his current active slot, so I am now trying that. However, when I try to enter the command fastboot set_active a on my computer, it returns the usage message (the list of commands) and when I try the command fastboot --set-active a, it says unrecognized option. 

I am using the platform-tools r29.0.5.
The command shouldn't have any spaces. The correct command will be:

fastboot --set-active=a


Or if you want to write it in short:

fastboot -aa to set the active slot to A.

fastboot -ab to set the active slot to B.