Best way to install content now is to open up the FAB window in your Unreal Engine window by going to the top and clicking Window > FAB (with FAB plugin enabled)
From there, you can add the content directly into your project and it even routes you to where the assets get placed.
I see that you tried this /\
I would log out and back into your account and shut down the epic games launcher.
If that does not fix it, I would contact the seller. I had this issue with a pack that had an invalid .uasset file.