Fsck.ext4

fsck.ext4 -f /
e2fsck 1.44.1 (24-Mar-2018)
fsck.ext4: Is a directory while trying to open /

The superblock could not be read or does not describe a valid ext2/ext3/ext4
filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
   e2fsck -b 8193 <device>
 or
   e2fsck -b 32768 <device>

See also

• ext4 e2fsck, fsck.ext4, superblock, inode, block size, mkfs.ext4 tune2fs, wipefs, resize2fs stat, extents, Review ext4 journalctl logs. Read-only file system, virt-resize, ACL