Thank you for bringing this to our attention. After further investigation it appears you are unable to see this option to Limit by Tag because your account has over 10k contacts in it. I apologize for any inconvenience this may cause! Being able to have this option in accounts with large lists of contacts is a feature request we have submitted in your account to the appropriate teams. In the meantime we suggest creating a new list for the tagged contacts and schedule the email to that list.