@Calc LLC Thanks for your patience on this.
I want to summarize this issue so that it could help the other community members looking for a similar issue.
Issue Details: Unable to create managed certificate in app service to domain www.MyDomain.com
Solution: On further investigation (you worked with support team) The auto-generated friendly name for the certificate already exists in a different region, but the system did not state that fact but give a general error message. Post adding a certificate by choosing a name other than the auto-generated friendly name helped to resolve the issue.
Please accept as Yes so that it can help others in the community looking for remediation for similar issues.