Add and verify an actual telephone number as a forwarding destination. It can be any mobile or landline/VoIP number, on which you can receive and respond to the verification phone call. After successfully doing that, test that calls made to your GV number do forward to that forwarding phone number, and you can answer the call there.
Log out of your Google account on your browser, clear the browser's cache, history and cookies, close and reopen the browser, and log back into your Google account.
After successfully doing that, then you should be able to delete the Chat forwarding destination and add it back again.