I'm using Photoshop 21.1.1 on Mac OS 10.15.3 and when I select File > Export > Save for Web, an error message appears. "Save for Web" error in Adobe: This operation could not be completed. An unknown error has occurred."
I don't understand the cause because I installed Photoshop from scratch, but resetting the PS preferences helped me. Press and hold Option+Command+Shift (Mac OS) immediately after launching Photoshop. You'll be prompted to clear the current settings.
I have a .psd file that can be exported as an animated GIF and saved for web and one that has the above error. I can provide both files for debugging. Please remove the solved with this problem. The problem still occurs! Thanks,