how do I get it to work?

You have probably bought a stolen code. Where did you buy it from?

https://www.gog.com/redeem

If it doesn't work it's either already used or it might have been revoked if it was bought with a stolen creditcard. That's a risk you take when buying keys from a key site.
The only legit keys are being sold by GOG.com or found on retail versions.
If you bought it on a key site, contact them. Otherwise contact support.

GoG deletes illegally acquired keys (search online for G2A / Kinguin grey market info and illegal keys / stolen credit cards etc).
If you bought the key from a non grey market reseller then I'm sure contacting GoG's support will solve your issue.