It’s probably that you don’t currently have write permission on the file (eg, maybe another application has opened Cratons.gpml and has a write lock on it).
I believe this error message pops up without crashing GPlates. However if it does crash then let me know.
It’s definitely an issue with writing to the filename in some form or another. Actually you mention your filename is ‘/Cratons.gpml’ which is in the root directory which you wouldn’t generally have permission to write to.