Ubuntu Error No Space Left on Device but There Are Still Space

It's possible to bump into Error: no space left on device while there are still plenty of space. One possibility is you run out of inode.

Check to make sure there are still disk space.

df -h

Check if there are still inode.

df -i

You might run out of inode faster than disk space when you have many small files.

Check for which directory have the most files.

sudo find . -xdev -type f | cut -d "/" -f 2 | sort | uniq -c | sort -n

The only solution is to delete unused small files or move them to another disk.