According to a few threads from the official Ubisoft support forums, all copies of Far Cry 4 purchased via online store G2A are being deleted from Uplay accounts. This means that everyone who bought the game from them won’t be able to play it anymore as of right now. We don’t know if this is permanent or not, as we’re still waiting for the publisher to release an official statement on this matter.
It’s not the first time PC games bought from G2A had this kind of issues, and sadly only customers are affected by this. It’s unlikely for the publisher to re-activate the game, so if you’re in this situation you should try contacting the seller for a refund.