Cannot delete device /dev/loop0

Are you sure that the device isn't busy? Have you ever tried fuser to determine an possible PID?

Precautionary get all possible information:

fuser -c /dev/loop0
fuser -d /dev/loop0
fuser -f /dev/loop0

Try to stop the process, which could use /dev/loop0. If necessary use kill -9 or try fuser -k to send the kill signal -> look at man fuser.

You may have to also use dmsetup to remove the device mapping. Easiest way, if it doesn't interfere with any other mappings is to use a dmsetup remove_all.