If Photoshop or another Adobe app reports that the scratch disk is full, it's most likely that the users' C: drive is filled. To help alleviate the problem, delete temporary files from the machine.
Close all Adobe applications if possible.
Open a PowerShell window. Hotkey: Windows + X, then I.
Paste: gci $env:temp | rm -recurse -force then hit ENTER.
You will see some errors from in-use files that cannot be deleted. Once the process is complete, you can close the PowerShell window.
Check the free space on C: to confirm that some space is available.