I'm sorry, I'm not sure what the issue could be based on the info you mentioned. All I can recommend is trying to delete your files and go through the install process for the game + DLC again. It's possible something just went wrong when adding the game and trying again will sort it out.
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Oh, that pop up is a generic response when it doesn't work. There's no real specific bug it's talking about. It could mean the game can't install because it needs more space, or the game won't install because a virus scanner has decided it's not safe or your settings don't let you install stuff not from the official app store.