Hi there,
We are sorry about the issue you are facing with Photoshop 2020 while trying to Save for Web.
Would you mind going to the System Preferences for Mac then navigate to Security and Privacy > select the Privacy Tab > Select Accessibility from the list on the left and add Photoshop to the list. Also, grant Photoshop permissions to the Files and Folders then restart the computer to check if that helps.
You can also try to reset the preferences of Photoshop to check if that helps. You can check the steps in the link mentioned below.
https://helpx.adobe.com/in/photoshop/using/preferences.html
Note: Please take a backup of your Actions, Presets, Plug-ins, etc. before resetting the Preferences.
Regards,
Akash