Injustice 2 released today, but not everyone has been able to access all the DLC they may have purchased. This isn't anything nefarious, not is it a colossal mess-up on the part of GameStop or any other entity. They didn't give you the wrong code or anything. The real issue is that Injustice 2 doesn't have the most straightforward way of accessing its DLC characters, specifically Darkseid, Injustice 2's pre-order bonus character.
Folks on the Injustice sub-Reddit have been dealing with this issue as well and have been sharing their solutions. Here are a list of some workarounds and maybe some issues that might be affecting whether or not you can access Darkseid.
Check Your Spam Folder
Mostly, this issue is affecting those who pre-ordered the digital edition of Injustice 2. For some reason, it seems the codes for the Ultimate Edition and Darkseid himself are different and came in separate emails. Players are reporting that the email with Darkseid has ended up in their spam folder from time to time.
Manual Separate Download
Another issue with digital pre-orders (possibly only on Xbox One), it seems you may have to manually download the content you ordered, as it doesn't just automatically download Darkseid and the rest of the DLC. Simply go to the PS Store or Xbox Store and search for "Darkseid," and you should be able to install him.
Restore Your Licenses (PS4)
Another issue people seem to be affecting PS4 Injustice 2 players, even when they finally get the appropriate Darkseid code and install the game is that it simply says "no content found." An easy fix that's built into the PS4 is restore your license. This actually works for any content that you download but fails to start. Simply go to settings/PlayStation Network/Account Management/Restore Licenses.
Time Heals All Wounds
A similar issue happened with Mortal Kombat XL, also by NetherRealm Studios. It seems the content in Injustice 2 downloads in tiers some times, and still lets you play even though the game might not be fully downloaded. Lo and behold, many players have waited a few hours only to find their previously locked content now fully accessible.
We will continue to search for other solutions.