Can't invite or create an Apple Search Ads API user

I'm trying to invite an Apple ID as a search ads api user, however when I try to accept the invitation from the invited Apple ID, I get the error "Unable to create Search Ads Basic account".

When I try to login directly to search ads using that account, I get the message "We're sorry. This Apple ID is no longer authorized for Apple Search Ads."

I've also tried this with a separate Apple ID account, and the same problems happened.

I've followed the steps in this article: https://searchads.apple.com/help/campaigns/0022-use-the-campaign-management-api

Is there something I'm missing? Do I need to create a brand new Apple ID for this to work?