Error: "Accounts must be entered before performing this action." appears when trying to add a new AWS account
When I click on "more" the following message is displayed "You are not subscribed to the EC2 service."
This error indicates that the account sign up process was not complete.
Review the setup steps of your account, as even not selecting your Support Plan for example, can be the cause for displaying of this error.
Then you should be able to add the account in Azure Cost Management.