First, in the pantheon, you choose a side and you are limited to 4 demigods until that tournament ends.
Second, there is a bug where some of those 4 are no longer available. You can either click back and click choose demigod again, or if that doesn't help, you can restart demigod. You should be able to choose one of your 4. There is no mechanic to limit what you choose, beyond the limit from the side you chose.
