Also didn't work for me this morning but worked this evening. I assume it was a transient issue. (Unless it was just a very confusing error for me having not yet finished the email confirmation first.)
With the latest software update, Rivian added the "EVGo Autocharge+" feature. I just checked, my R1T is there and I used the VIN. I think I did this over a week ago.
I have yet to charge outside of our house and the 2 RAN stations (Sacramento, Manteca), so I can't say how it will work.
The app accepts my VIN but I haven't been able to complete enrollment. I've tried at 3 different EVGo stations and when i can even get to the point where it's "syncing your vehicles unique identification to your account" it fails. I have a ticket open with EVGO as it goes up through the levels of tech support. I use EVGo at least once a week so it would be a nice feature.