Don't worry if you're having trouble running that digital version of Dying Light that you bought on PS4, you're not alone. There's a pesky bug affecting digital versions of the game on PS4 where the system thinks that the game has already been downloaded and installed even when it hasn't. I punched in a download code for the game yesterday and instead of a download button, it simply said "Start" as if it had already been installed.
Annoying, but not insurmountable.
Basically, if you're faced with the same issue, simply log into the PlayStation Store on another device, app or browser and opt to download the game remotely. Once you've logged in, there should be a button underneath the game thumbnail that reads "Download to PS4". Click that and it'll start downloading properly on your machine.
For a longer explanation and more info, click here.