It sounds like a service tier change is currently in progress on your Azure SQL MI (Managed instance). This can temporarily prevent other operations, such as restoring a database.
You can opt to wait for the operation to complete. You can check the status of the service tier change operation in the Azure portal. After confirming that the service tier change is complete, try the restore operation again.
If the service tier change seems stuck or takes an unusually long time, consider reaching out to Azure Support for further assistance.