@Patrick Sims Welcome to Microsoft Q & A Community Forum. You were trying to back up a Trusted Launch VM , however you were prompted with an error "Standard policy not supported for Trusted Azure VMs" . Kindly note that Trusted Launch VM is supported with Enhanced policy only. To enroll your subscription for backup of Trusted Launch VM, write to us at askazurebackupteam@microsoft.com. For more information, refer to this document.
To change the trusted launch vm configuration, in the left menu, select Configuration under the Settings section. You can enable or disable Secure Boot and vTPM from the Trusted LaunchSecurity type section. Select Save at the top of the page when you are done. Which I assume you are able to do it. But can you elaborate what you meant by disabling the trusted vm ? Are you trying to shut it down?