have just added the default variant id dropdown to one of my existing ce’s. edited the control and added the id of the correct content type. however, when creating content the field has no options to choose from.
Check whether you have the page variants for the content type you have added?
The easy way to find is to get the URL you gave in the choices and add the content type id parameter to it and execute it in the browser and check the results.
In 5.7 the variant dropdown only shows templates that have been assigned to the community you are currently logged into. Check in the system tab that the variants have been correctly assigned.