umount: target is busy

Created by: antonok-edm

Randomly and suddenly, I got the following error from pmbootstrap install. zap and shutdown also subsequently provided the same error.

(005120) [15:05:39] % sudo umount /home/antonok/.local/var/pmbootstrap/chroot_native/mnt/rootfs_asus-grouper
umount: /home/antonok/.local/var/pmbootstrap/chroot_native/mnt/rootfs_asus-grouper: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

Couldn't find anything useful in lsof or fuser, but I am also not experienced with these commands. No clue what might have contributed to this happening, but I did not use chroot manually.