You can redeem a reward as long as the reward is unlocked and you have the required points needed to redeem that reward. Visit the Rewards section of your ReceiptPal app to see which, if any, rewards are unlocked and the number of points needed for redemption.
Make sure you've got the most recent version of the ReceiptPal app in order to access the latest reward options. Depending upon your device, go to either the Apple App Store or Google Play Store to download the latest version.