I had this problem a few weeks ago…exactly
you have to do this on the website…can’t add a license from the phone app.
i waited to off hours …after mid nite…loaded the code it accepted it…silently…
apparently either the license server is bogged down or the firewall in front of the server is bogged down.
checked for security features…no obvious change
sumbitted a tictet
had to go to the org, and find a ribbon, click on it…so that the subscription can be applied.