Do not use df on btrfs in this situation !
It also says:
Tip: Check how close you are to “out of space”.
Look at Device unallocated: (not at Free (estimated): ) 1
as root :
btrfs filesystem usage /
Did you try to list the mounts (rw / ro) ?
mount -t btrfs
First you need to know the exact cause of the problem. Then we can discuss the solution. At least that’s my approach.
And where is your inxi ?