Cause: You have used all of your license's site activation slots, or there is a typo in the key.
Fix:
Make sure you are not accidentally entering extra spaces before or after the license key. Copy-paste carefully.
If you have used all slots, log in to the WordPress admin on a site where the plugin is activated but no longer needed.
Go to Settings and click Deactivate License to free up a slot.
Now try activating on your new site again.
If the old site no longer exists, contact support at onlineimsupport.com to have the activation manually removed.