This issue has been resolved in the latest InDesign version 15.1
Adobe InDesign team
An error occurred while saving the commentAdam Fuchs commented
I ran into this today (5/2/2018). First of–screw Gatekeeper and High Sierra permissions. I get the theory behind, but don't roll it out until its actually ready.
Secondly—go into terminal and run:
chmod 777 Caches
chmod 777 Preferences
Restart your computer and open inDesign.
NOTE: This process has also worked for me with numerous other programs that don't get along with High Sierra in case you run into others. Always set 777 on Cache (not sure why it gets reset to not 777) and then whatever folder you get the error on.