If you've been sold a code for post-apocalyptic adventure Metro Exodus, you might want to check where it came from. According to publisher Deep Silver, a number of keys have been stolen from the factory where physical key printing for the game had been taking place, before it became an Epic Games Store exclusive. In a post on Steam, the company explained that physical keys don't have the necessary files to run "as they were not meant to be released."
As third party sellers have been selling Steam keys for a game that's no longer even on Steam, they're obviously illegal. Deep Silver has deactivated the stolen keys and recommends that, if you've been affected, to "contact the seller who sold you the key and demand a refund."
The only supported key sellers for the game were HumbleBundle and the Razer store -- where pre-orders were honored. For now, though, you'll have to get it through the Epic Games Store, or wait for Metro Exodus to return to Steam after February 14, 2020.