Can't activate Install Where do I get authentication code?

Please have a look at the plugin’s description:

To use this plugin you will need your own Piwik instance. If you do not already have a Piwik setup, you have two simple options: use either Self-hosted or Cloud-hosted.

Self-hosted: You have to download & install Piwik on your own. See https://piwik.org/ and https://piwik.org/docs/installation-optimization/

Cloud-hosted: You can get a paid Piwik Pro (“Piwik Cloud”) account and use this instead of your own instance, see https://piwik.org/hosting/

If you installed Piwik (or decided to use the cloud), you can follow this description to get your auth token: https://piwik.org/blog/2015/05/wordpress-integration-wp-piwik-1-0/