In order to use your Certain Arrive credits, your subscription must be active. To view your billing and subscription information, navigate to your username in the top right corner and select Billing from the drop down. The billing page will list which plan you are on, and when your plan is set to renew. Please upgrade/renew your subscription accordingly.
Please note, your renewal does come with a number of check-ins based on your Subscription Plan. Look at your renewal date before purchasing a large number of extra credits.