can't save images

Unfortunately you haven't given us any information about your system. Therefore, all proposed solutions are a look into the crystal ball

Please provide a copy of your 'System Info'. This can be get from the Photoshop menu item Help > System Info. There's a copy button in the System Info dialog. Press this button and paste the info into a text file and attach it to your next forum post.

More and detailed information would be helpful to identifying the problem.

Occur the issue only with this Save for Web command, or also with other Save commands ?

Do you use old Save for Web (legacy) command or any other command to save web files ?

Are you aware of Mac applications requiring rights to access certain files and folders ?

Most of the apps handle that automatically (upon installation), but sometime something went wrong. Check it, depending on your  MacOS version, in the Settings, under the category Privacy & Security, the tab Files & Folders.