How to open a support ticket for Power Bi?

If you have a Pro license you can open a Pro ticket at https://admin.powerplatform.microsoft.com/newsupportticket/powerbi
Otherwise you can raise an issue at https://community.fabric.microsoft.com/t5/Issues/idb-p/Issues .

Pro ticket response time varies depending on your company's support contract.  First level support is a Microsoft partner who will contact you according to your stated preferences (phone/email) and will arrange for the initial triage call.