You are trying to edit an existing email template, and it has the status of "Approved with Draft" However the draft is not showing up in menu tree and when you try to edit and save, you get the error "Not Allowed – Template already has a draft."
Refresh the browser or check is the draft is opened in another window.
Try logging out and logging back in
Try using a different browser to check it further
Check if there is a template with the same name that exists elsewhere (Design Studio)
Rename the template and check if that is working
If the issue persists, please reach out to Marketo Support for further assistance