Rubik recently made this live, so there might be some small bugs that come along with it. There are a few solutions and possibilities to your problem that Rubik has provided, which consist of;
They are logged into a Steam account that is already linked to another Discord
Their Discord account is already linked to another Steam account
Their account is Steam OAuth banned (extremely rare)
They are device-banned from Discord on an alt (rare and they won't even get to the verification screen)
If they get a message that they're already linked, they need to contact me to unlink it.
If their account is OAuth banned they need to use a different account
If they are device/ip banned by Discord, they need to appeal on the forums with the ID of their other account
Also, yeah... unfortunately a few bad apples ruined the bunch.