User has already purchased subscriptions for Core, but still is in the trial mode and sees the Subscribe button.
This can happen if you have created multiple companies and purchased subscriptions for one of them. You can check all your companies in the Manage Companies screen. The companies that are in trial mode show the status Trial while the ones that have active subscriptions show the status Active. You can switch to the Active company and even set it as the default one. Check the Core Help for more details.