Try to remove it from the dom0 terminal e.g.:
qvm-remove -v backup-appvm
Maybe it’ll print the errors if it’ll encounter any.
You can also try to run this command to make sure that the deleted data is discarded:
fstrim -av
Now it is deleted and vm-pool data has 35.5% now we I’ll see
Appvms can be start again but Autostart of sys-net sys-usb at boot doesn’t still work
Did you try to reboot one more time?
There could be some failed volume revisions for the first boot that could be fixed after the first qube start.
Or is it failing to start after you tried to reboot a few times?
Then check the journalctl -b in dom0 to see the errors why they fail to start.