Ok. I have just downloaded the latest Sketchup 2019 .3 for Mac, thinking this would stop the problem.
The problem being that SketchUp cannot save the file. Not Save or Save as. I have tried shutting down the app and it prompts to save the file and then it can’t. I dont know what has changed from last week. All the forum chat doesn’t seem to address this specifically. This is the latest SketchUp 2019, version 3 on a MacBook Pro, sporting Mojave 10.14.5. I can’t save any files on Sketch Up.