Flashing TWRP.img on Xiaomi Mi A3 to root the device does not work H1@all
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.

I downloadet: 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.
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
if you active slot is &quot;a&quot;, change it to active slot b. than flash twrp.img and reboot the phone while press volume key &quot;+&quot;. now you are in the recovery and you have to reinstall it in the slot a, for do this change active slot to a and press install and select again twrp.img