Thank you for sharing your issue on Microsoft Q&A.
I Understand that you are getting a 403 error while assigning RBAC in Azure portal but not via Terraform even though you have owner role.
The error message indicates that the scope is either invalid or the client does not have the required permissions to perform the action. Since you mentioned that you have an owner role, you should be able to assign users under RBAC. so, double-check that it exists and that you have access to it. If possible, please share your Terraform code.
If the issue continues, provide more details about the error so we can troubleshoot further.
I hope this information is helpful. Please feel free to reach out if you have any further questions.
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Best regards,
Harshitha Eligeti.