customer use several subscriptions and when running the role assignment for the reader role this was done on another subscription.  After assigning it on the correct subscription the Azure resource in the RedHat portal was created successfully 

"az role assignment create --assignee "<your_Client_ID>" --role "Cost Management Reader" "

Maybe a small enhancement in the documentation will prevent also others for making this mistake, by explicitly adding the "--scope" to the command

like:

"az role assignment create -assignee "<your_Client_ID>" --role "Cost Management Reader"  -scope <scope ID>"

 https://access.redhat.com/documentation/en-us/cost_management_service/2023/html-single/adding_a_microsoft_azure_source_to_cost_management/index#configuring-azure-roles_adding-an-azure-source

https://learn.microsoft.com/en-us/cli/azure/role/assignment?view=azure-cli-latest