The Rewards tab can hide if your device's time settings are out of sync with server time. Here's how to fix this:

1) Go to "Settings" on your device
2) Select "General" and then "Date & Time"
3) Make sure that "Set Automatically" is ON and that your Time Zone is correct.
4) Fully exit out of Choices and reopen the game.

If you are on mobile data, you may also need to turn it off and connect to WiFi for a few minutes. Defective cell towers nearby can sometimes broadcast inaccurate network time information.