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.
I confess that I don’t know a solution; I am not experiencing this when running SketchUp 19.3.252 on Mojave 10.14.6. Are you trying to save to a networked or cloud drive? I don’t know the technical explanation, but there are known issues with such services somehow interfering with how SketchUp saves files.