To make it clear to everyone, the 'free''keys posted by ignis will not work for you.
They are KMS client product keys or Microsoft Generic Volume License Keys. You can find them here: https://learn.microsoft.com/en-us/windows-server/get-started/kms-client-activation-keys
Obviously posting a link without understanding what they are posting.
To use the keys listed in the link supplied by ignis (which are GVLKs), you must first have a KMS host available on your local network. If you don't already have a KMS host, please see how to create a KMS host to learn more.
If you want to activate Windows without a KMS host available and outside of a volume-activation scenario (for example, you're trying to activate a retail version of Windows client), these keys won't work. You will need to use another method of activating Windows, such as using a MAK, or purchasing a retail license.