Unable to create secrets in Azure Key Vault if using Azure role-based access control

I'm really new to Azure but trying to learn - so apologies if this is a daft question. I've started the free trial (which gives me some credit to start with), and I'm trying to create a key vault. If I specify "Vault access policy" under access policies, it works great and I'm able to create secrets. I'd like to use "Azure role-based access control" though instead. If I create a key vault using Azure role-based access control, I get a message when trying to create a new secret which says "The operation is not allowed by RBAC. If role assignments were recently changed, please wait several minutes for role assignments to become effective." I am logged into Azure as the Service Administrator, so I don't think it's a permissions issue. I have left it overnight, and it still displays the message, so I don't think it's a matter of not waiting long enough for role assignments to become effective. I have tried creating the key vault in different regions and get the same results. Note that this is happening when I create a key vault using Azure role-based access control from scratch (i.e. I am not changing it from one to the other or anything like that.) Does anyone know what I'm doing wrong? Thanks for any help Andrew