[2021] Can't buy premium cosmetic in-game

I have to ask, have you tried restarting Vermintide 2? Otherwise, try the ‘Verify Integrity of Game Files’ option:

1. Right-click Vermintide 2 in your Steam library sidebar
2. Select ‘Properties’
3. Select the ‘Local Files’ tab
4. Select the ‘Verify the Integrity of Game Files’ button
5. When complete, close and reopen Steam