Maybe it cannot find the directory of the BP? Have a look at this page: A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums
Just a quick example, here’s what I do to get an FString to my Content directory:
FString thePath = FPaths::ConvertRelativePathToFull(FPaths::GameContentDir());
Also does AddOnScreenDebugMessage() work in Packaged buildings? I’d assume they don’t, but could be wrong.