My Photoshop CC 2021 suddenly does not allow me to save my files using the Save As command. The file/s are not overly large. The same problem was there around end February 2021 but a person from Adobe solved the issue on-line.
Without any system info and other technical details we can't tell you much. Simply sounds like you have permission issues on your computer, be that for the file saves themselves or temporary data PS uses and generates. As such checking your user folder, documents folder and PS swap file locations may be a good place to start.