FAILED (remote: 'Fastboot command is not allowed

Moving Fastboot to Userspace fastboot set active slot If this flag is not used, slotted partitions will default to the current active slot -a, --set-active Sets the active slot If no slot fastboot_ error_ failed to identify current slot slots ourselves using a Fastboot command Changing Android's Active Partition Slot

fastboot set active slot fastboot commands :- https How to Check and Change Current Active Slot on Android