To have both General Admission and Reserved Seating, you must start by building your show with the Reserved setup.
When you are at the following screen during the show building process and are prompted with the question "Will seating be Reserved or General Admission?", choose "Reserved-Seating is Assigned".
After you have finished completing the rest of your show build and after publishing, go to the Show Details tab from the left side black dashboard. Note: At the top of your show, it will reflect as Reserved. This is okay!
Follow this process to add GA sections to your show. Please note that you need to create GA sections on your seating chart after publishing. These seats will not be selectable individually. They will simply allow a patron to puse GA tickets.
Congratulations! You have successfully added General Admission tickets!
On your shows ticketing page, it will now reflect that you have both Reserved Seating and General Seating.