I had the issue, It's when a chair is inaccessible i think. You need a gap. Same for doors.

Also check the entrance sign and various doors :)

I just got this and had no idea why. I wanted to create some columns/pillars out of brick walls inside the restaurant and these were the problem. Any enclosed area reads as an error. It's a bit unforgiving.

Originally posted by im_Brandon:

It says Part of my restaurant is inaccessible, and to check the highlighted tiles. the problem is I have no highlighted tiles. Help pls

The game can't handle corners. Example, place two storage shelves with a corner of the room being empty and the game will complain because that one square with nothing on it can't be accessed.