The following tech tricks have worked when you can’t access your course after purchase:
Backing/Signing out and refreshing your browser before starting over.
Clearing the cache on your browser. (This is different from history.)
Do not use a phone for the certification, not even the purchase.
Chrome and Firefox have tested well. Issues have been reported with Safari and Edge.
Try another browser if the first one still gets stuck.
Make sure your internet provider allows our site. (Schools and institutions have strong filters and could block our courses. Several have come back and told me that using the internet at home worked when the school internet didn’t.)