When I try to log into Steam through the link after pressing Purchase VIP I get the image below Image: https://imgur.com/a/YyNaL
Did you try linking your account outside of the store? You can do so and try it here first. In addition, is GMOD steam shared to your steam account?
Currently, we are looking into the issue. I'll tag @Highwon to see if he has additional input on if the steam link or store is broken.
Problems like this were happening before Opalbot was even around (even before I was around) lets just wait till either the issue resolves itself or Highwon can fix it.
I believe she doesn't play a role in these transactions. The current theory is a steam link broke with the recent update.