Even though requested the release preview account, can not access the RP account.
Solution – One possible reason may be with the role permission. Initially, only users with an Administrator role have access to the Release Preview account. A user with the Administrator role can give Release Preview access to other users as needed for testing.
So, log in to the account using the Admin role. In the dropdown, the RP account will be available. Provide access to the custom role for the access.