I get the above message when downloading Diablo 2 from the official installer I got from account.blizzard com games#classic-game-accounts. It also says elsewhere on the downloader “This download is not authorized” but I haven’t even been asked for a cd key yet.
What is going on?
Hello Malagant!
Please follow these steps 
I solved this issue by opening “internet options” and restoring the security slider for “Internet” from high back to Medium-high. This is not default but easily overlooked if you ever changed it.
I am having the same issue, there is no resolution by running in admin mode or any other registry edits or deletions.