Over 250GB available BUT not enough space error

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If the snapshot take space, you can check them like that:

export UUID=$(findmnt -n -o UUID /)

export MNTPNT=$(mktemp -d -t "btrfs.XXX")

sudo mount -m -U "$UUID" "$MNTPNT"

for x in $(sudo btrfs subvolume list "$MNTPNT" | cut -d' ' -f9); do 
   echo ">>> Subvolume: $x"
   sudo btrfs filesystem du -s "$MNTPNT/$x"
done

sudo umount "$MNTPNT"

And a one-liner:

export UUID=$(findmnt -n -o UUID /); export MNTPNT=$(mktemp -d -t "btrfs.XXX"); sudo mount -m -U "$UUID" "$MNTPNT"; sleep 3 && for x in $(sudo btrfs subvolume list "$MNTPNT" | cut -d' ' -f9); do echo ">>> Subvolume: $x"; sudo btrfs filesystem du -s "$MNTPNT/$x"; sleep 3; done && sudo umount "$MNTPNT"