How do I activate the API key on a WordPress multisite?
You have to activate the plugin as the network admin and then activate it manually on each individual site in the multisite. Once you have done that, the Settings menu appears and you can add the API key for each individual site. As an alternative, you can edit wp-config.php and add this line define('SHORTPIXEL_API_KEY', 'APIKEY') where 'APIKEY' is the API Key received upon sign up. If configured that way, the API key will be used for all the sites of the multisite but will only be visible on the main site’s Settings page, being hidden for the rest of the sites.