- is the file opened in some other application? Maybe the fact that you sync is giving problems because in Photoshop it seems the file is in use by another application, this can explain if the problem is intermittent, when Drive is syncronizing, Photoshop cannot save;
- did you check if the file is locked? Get on the file, right clic with the mouse, choose from the drop down menu Proprieties and check if the file is "read only".
Sometimes I had this kind of error with my students in the class. I fixed it making a new document with same size of the original, copying all the layers in the new one and saving it again.
Unfortunately, these steps presented do not work when everything is already unlocked, no program using the same file, nothing. From a closed and unattached file, I receive the same message and from other postings I've tried, nothing is working. Even in the Library, the Adobe file doesn't exist.
Hopefully, Adobe will get a fix to this issue and update everyone with a new patch to correct this issue. It is THEIR program with the issue, not users.
I turned off the 'sync' from my cloud ... it should work fine.