umount: /: target is busy

When I ran the command

umount /dev/mapper/ubuntu--gnome--vg-root I I got :

umount: /: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

I tried some different options with the command lsof and fuser, but it didn't work so far.

Does anyone have any idea how to resolve this issue?

Please try this commands:

umount -l /PATH/OF/BUSY-DEVICE
umount -f /PATH/OF/BUSY-NFS(NETWORK-FILE-SYSTEM)

Reference: https://stackoverflow.com/questions/7878707/umount-a-busy-device

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