This is a random irritation that frustrated me for a long time with Photoshop Elements on Mac. I'd make changes to an image and receive this error upon trying to save: "Could not save [file name] because the file is already in use or left open."
I searched forums for the answer to this problem and settled on a workaround of saving-as another file name, then deleting the original version. Doable but clumsy.
Till the day I had a eureka moment that was so simple and so satisfying.
Want to know where the file was "in use or left open"?
It was on my Finder.