Account Recovery "Not Available" error

My mother got a new iPhone and a new phone number (she switched carriers). She successfully entered her new phone number into the Apple ID settings on her iPhone. The old number is no longer listed there at all. She subsequently updated her Apple ID password, and now Apple wants to send a verification code to her old phone number, which she no longer has access to. One option is to request a code from one of her trusted contacts via the Account Recovery process. I am the trusted contact for my mother. When I go into my settings to request a code for her to use, it works fine until I get to “Generate Recovery Code” for her name. Then I get a “Not Available - error description not available” message. This was supposed to be our fail-safe. Why in the world am I getting this error message?