As with their previous promotions, it should be free for a week, however I haven’t seen any confirmation about it. Also might require subscription to their newsletter (from which you can unsubscribe as soon as you’ve completed checkout). I also had an issue with redeeming their previous free game (“Page not found” at checkout), I’ve contacted support just now and they said my home IP range is blocked for some reason, so I should try from mobile data instead of home WIFI/internet. Which I did, and it worked. VPN didn’t work for me, since they’re also blocking most VPN IPs, but at least they notify you about that if you try.
[Free Games] [Expiry Unknown, probably about a week] Peaky Blinders: Mastermind