What does "Check the instructions that came with your code" mean?

This error means you're trying to redeem a Trial key on a Microsoft account that has already had a subscription before. Trial keys only work on brand new accounts that have never had Xbox Game Pass, Xbox Live Gold, or Game Pass Ultimate — not even a free trial. To fix it, create a new Microsoft account at https://signup.live.com and redeem your code at https://redeem.microsoft.com.