Most importantly, make sure that you’re redeeming the key on the correct platform. Remember to always check the full name of the product - the platform or the way of activation is always indicated there.
For example, this is a game key for GOG platform:
Keep in mind that some products can’t be activated on Steam, Origin, Uplay, or Battle.net, but on a developer’s own platform - such keys are usually described as Digital Download.
For more activation details, scroll down the product page and find an appropriate tab.
Next, make sure that the product description doesn’t mention any restrictions that could affect the activation - regional restrictions or other product limitations are always highlighted in bold or red font in the product description.
If you are activating the key in a correct way and there are no limitations but the key’s still not working, please contact us via Resolution Center.
Do not forget to attach screenshots to your ticket.