You can, as vi (or, more likely, vim) is saying force a write by doing:
:w!
Alternatively, write the file to an alternative location:
:w! /path/to/other/location
To check where your backup files are being written (normally):
:se backup? backupdir? backupext?
Try to touch a file in that directory and see if your Operating System gives you an error to enlighten you as to why your editor cannot write there.