Contacted support for help, and after some extremely basic things they had me do, they now say they can not help and sent me here.
I have downloaded the classic WC3 and when I try to log in to my Battlenet account in order to verify that I pre-purchased the Reforged edition. I keep getting a message “Failed to authenticate 6.9”
I’m having a similar issue. I pre-ordered Reforged towards the end of 2018, and for awhile was able to play classic W3. Earlier this week, I tried logging in after having not logged in for ~3 months. I kept getting a message saying “An Error Occurred”. It may have something to do with the 3 month account check they do for classic games, but it could also be something different, because it’s like the game client doesn’t recognize that I have a license to play.
I think I figured out what to do. Your game might need an update. Open the Warcraft III Launcher (not the game application, it’s a separate file), and see if you get a prompt to update.