Tha's seems correct you can't add users to admin group. Use the below REST API call to create or update the users. If you are the owner/creator of APIM instance then you can update your email under Notification template. This will allow any update the admin email

https://learn.microsoft.com/en-us/rest/api/apimanagement/user/create-or-update?view=rest-apimanagement-2024-05-01&tabs=HTTP