It is available but I’m almost certain it is just in your service plan settings. These options can be disabled at the plan level, so go in on your reseller admin, look on your plans. Fine the plan with the service domain on it, edit that plan and look for ‘Custom DNS Records’ it is probably NOT checked, check it, then go down select ‘keep prices as is’ or some such wording near this, and submit/save the plan, then go back in your service domain panel and you should see the edit dns functions.