Try the request in a Send an HTTP request action of the Office 365 Outlook connector
https://learn.microsoft.com/en-us/connectors/office365/#send-an-http-request
Below is an example, if that helps?
1. Flow setup
https://graph.microsoft.com/v1.0/users/@{outputs('Get_user')?['body/userPrincipalName']}/messages/@{variables('MessageId')}?$select=categories
2. Test result