Expected exactly one role management policy assignment but 0 were found

I am getting the following error in Entra when I try to assign a custom role to a user:

Expected exactly one role management policy assignment but 0 were found

What I have done:

entra.microsoft.com → Entra ID: Roles & admins → New custom role

Basics
* Role name: User Directory Reader 
* Description: Provides read-only access to user directory information. Intended for security analysts and auditors who require visibility into user identities without modification permissions. 

Permissions
* microsoft.directory/users/standard/read 
*microsoft.directory/users/memberOf/read 
*microsoft.directory/users/manager/read 

When I click + Add assignment I get the error "Expected exactly one role management policy assignment but 0 were found". 

I have trial license for Privileged Identity Management (Entra ID P2).