In-game it tells me "platform not supported" (or something like that) when I try to sign in. I have my steam key, what do I do with it?
If you don't use Steam (some people literally can't - it's been a known health hazard for over 18 months), get a refund. They've functionally taken the game you bought away from you aren are trying to give you a version you don't have access to in its place.
If you're ok with using Steam, you can use Steam. Alternatively, you can play without an account and accept the (excessive) limitations that comes with.
https://steamcommunity.com/groups/SteamClientBeta/discussions/3/2962768084999962...
Here, have a source where one of the Steam moderators is acknowledging the problem because of how widespread the complaints about it were getting.