If you don't have Showtime, a black box will be in it's place, as it has been removed from game and will not function.
It will look like this: (Click image to see larger view)
✽ The game does not replace the stadium automatically. If you do not have Showtime, it will need to be replaced. There are two to options to fix this:
1. Edit the lot, delete the stadium shell. Go into Build Mode > Community Objects > Buildings tab and Filter by Sims 3 and look for Llama Memorial Stadium and place it on the lot.
2. Load up the original Sunset Valley and save a copy of the original stadium to your library and place it into your SV2020 save file so that you can have a functional stadium.
✽ Due to replacing the stadium, you will need to have your Sim join the Sport Career again.
The Stadium from Showtime was added to the SV2020 saves as it's not only a stadium, it's a big show venue.