Hi there, sorry to hear you are having difficulty purchasing Steel through the in game store. As you are advising you launch your game client through your Steam games library could we request you attempt the following steps :
1) Close For Honor, your Steam client and also log out and close your Ubisoft / Uplay games client.
2) Locate and access the folder: D:\Steam\appcache\httpcache and delete it. (This folder might be saved somewhere else depending on your set up).
3) Re-launch your game from Steam, logging in to your Ubisoft account again when prompted.
If you still continue to experience this issue, could you please provide an image or short video clip showing this specific error in process.
Apologies for any confusion or inconvenience this may have caused.