Comment 28 for bug 1768038

This worked like a charm for me:

https://www.reddit.com/r/xubuntu/comments/8w23gu/how_to_fix_the_suspend_on_inactivity/

Sorry, if already known, haven't read all links. It solved my "permission denied" problem. Maybe you can extract a fix out of this workaround. I would help, but my knowledge is pretty limited.