Flashing TWRP.img on Xiaomi Mi A3 to root the device does not work

Working on Ubuntu 20.04. I want to root my device to uninstall some apps (.apk) and possibly also install Ubuntu Touch. I want to uninstall almost all Google apps because I don't have a Google account and I don't want to. Without a Google account, you cannot use most Google applications (Gmail, YouTube, Drive...). A few months ago when I bought the device, I unlocked the bootloader, but I didn't have time to root it. Now I want to root it. I followed these TWRP for Xiaomi Mi A3 instructions and cannot flash or boot the TWRP.img. 1. I downloaded: twrp-installer-3.4.0-0-laurel_sprout.zip and twrp-3.4.0-0-laurel_sprout.img renamed it to **twrp.img** and **twrp-installer.zip**. 2. I copied the files to the device in **/sdcard** ---- adb devices adb push ~/Downloads/twrp.img /sdcard adb push ~/Downloads/twrp-installer.zip /sdcard adb reboot-bootloader ---- fastboot devices ---- fastboot flashing unlock ... FAILED (remote: Device already : unlocked!) finished. total time: 0.000s ---- fastboot flashing unlock_critical ... OKAY [ 0.026s] finished. total time: 0.026s ---- fastboot flashing get_unlock_ability ... (bootloader) get_unlock_ability: 1 OKAY [ 0.000s] finished. total time: 0.000s ---- fastboot boot twrp.img downloading 'boot.img'... OKAY [ 1.174s] booting... FAILED (remote: unknown command) finished. total time: 1.175s ---- fastboot flash recovery twrp.img target reported max download size of 804261888 bytes sending 'recovery' (39892 KB)... OKAY [ 1.172s] writing 'recovery'... FAILED (remote: (recovery_a) No such partition) finished. total time: 1.174s ---- Did I do something wrong? How can I root the device? To recover the device follow this instructions: https://c.mi.com/oc/miuidownload/detail?guide=2
--EDIT-- fastboot getvar current-slot current-slot: a finished. total time: 0.002s ---- fastboot --set-active=b Setting current slot to 'b'... OKAY [ 0.039s] finished. total time: 0.039s ---- fastboot flash boot twrp.img target reported max download size of 804261888 bytes sending 'boot_b' (39892 KB)... OKAY [ 1.181s] writing 'boot_b'... OKAY [ 0.207s] finished. total time: 1.388s Press and hold **volume up button** on device and enter the command **fastboot reboot**
Back in Fastboot ---- fastboot --set-active=a Setting current slot to 'a'... OKAY [ 0.039s] finished. total time: 0.039s Press and hold **volume up button** on device and enter the command **fastboot reboot** Do not press any key boots in Android-OS