You might have a separate staging website for testing and development purposes. Your staging website should also receive plugin updates and therefore needs a valid license key.
Are you using one of the below descriptions in your staging URL? In that way, you can enable the same license key used for your live website, without needing to pay for an extra domain.
'.cloudwaysapps.com'
'.dev.'
'.dreamhosters.com'
'.flywheelstaging.com'
'.kinsta.cloud'
'.kinsta.com'
'.lightningbasehosted.com'
'.loc'
'.local'
'.localhost'
'.localhost.'
'.mybluehost.com'
'.myftpupload.com'
'.myraidboxes.de'
'.pantheonlocal.com'
'.pantheonsite.io'
'.savviihq.com'
'.stage.site'
'.staging'
'.staging.'
'.test'
'.test.'
'.tribe'
'.wpengine.com'
'.wpsc.site'
'.wpstagecoach.com'
'beta.'
'corpwebstg.'
'dev.'
'devsite.'
'local.'
'localhost'
'pantheon-dev.'
'pantheon-live.'
'sandbox.'
'stage.'
'test.'
'upgrade.'
'webdev.'
'wpdev.'