I've never encountered this issue before -- I can see the source code when simply viewing the Cloud Function, but when I go into edit I just get this error message:
I wrote the function and am an owner of the GCP project, so there shouldn't be any permissions issues. I was not, however, the last person to edit the CF, so maybe there is an issue with that?
The GCP CF common errors page does not seem to have this issue documented: https://cloud.google.com/functions/docs/troubleshooting
Thanks