I'm logged into my GOG account and am trying to buy a couple of games but don't seem to be able to add anything to the cart. When I click "Add to Cart", the cart icon changes from "0" to "1" but then goes back to "0".

I've disconnected from my VPN and tried both Chrome and Waterfox. What am I doing wrong?

Same problem here pal, i want to buy witcher 3 and i can't add it to the cart

You're not doing anything wrong, it's not just you. The site was slow/broken for a couple hours or so. Probably the cart and other features are not back up yet, the front page is still almost totally devoid of content for me.

lol, I have wicher3, trying to get 1 & 2. thanks, I wish they had some sort of system status so I wouldn't have ask dumb questions.

This hasn't been fixed. I'm having the same problem right now. Can't use the ongoing sale because the games can't be added to the cart. It jumps right back to 0...
The same behavior applies to wishlisting. When I click it, it gets unmarked right away.
I tried different browsers, gog galaxy, even different devices (pc and android phone) . The problem persists.

So the suggestion of clearing your cache to refresh any JS won't help then.

Is it a base game or DLC or a music pack?
Does it happen with something that's on sale or not on sale?
Does it only happen on one game?
Is it a game you already own (or a DLC with one you don't own) and thus it would be a gift-only?
Did you start with an empty cart? Or did you already have games in it?

Some of these questions answered may help locate the bug. Though it could just be an intra-server issue of one system getting the order, forwarding it, and the other failing to get it thus when it refreshes it returns to the previous state and has nothing to do with code and everything to do with fixing the servers.

Or maybe region locked games?
See: https://www.gog.com/forum/general/regional_locked_games_on_gog_v2/post579

If someone's in a region where a certain game is unavailable for purchase, they also won't be able to see that game in the store.

Under normal circumstances that's true, but the sale has come with some breakage, e.g. I'm not in Japan but can currently view the Japan-only version of Mad Max at https://www.gog.com/game/mad_max_jp and if I try to add it to my cart, the cart does instantly empty itself.

You did read the post I linked? At the moment I can find and access the store page of region locked games, but once I add them to the cart they are removed an instant later. Exactly as bulletchief described.

Sorry, no, I didn't click through. I apologize for the confusion.