We apologize for the issue you are facing while trying to use Save for Web in Photoshop Elements 17.0 on your Mac.
Would you mind sharing the macOS version you are currently using?
In case you are using macOS Mojave (10.14.x), please follow the steps mentioned below to check if it helps.
Click on the Apple logo on the top left and select System Preferences.
Navigate to Security and Privacy > Select the Privacy panel > Select Accessibility from the list on the left.
You would see a list on the right once you select Accessibility. Please check if you see Photoshop Elements Editor in the list. If you do not, please unlock the screen using the lock icon on the bottom left and add the Photoshop Elements editor from the applications folder and ensure that you see a check mark before its name.
Then restart the computer and try using Save for Web in Photoshop Elements again to check if that works.