Regardless of the platform you will find that some rewards are free, some need to be unlocked using Units. The amount of Units needed will be shown above the Unlock button (we'll explain how to get there below). If you have sufficient Units available, you will be able to unlock the reward.
You may also see small padlock symbols on some of the rewards. This usually means that you first need to fulfill a certain requirement, for example play a certain mission in-game, before this reward becomes available to you.
We have summarised the steps in dedicated articles for PC, PS4 and Xbox One.
Alternatively, if you would like to unlock rewards via the website, please check out this article.