Maybe this thread will help you. Let us know if it doesn’t…
PS: “Try logout, restart, and login back”
PPS: “Have you subscribed to the specialization? If not, one possibility is that you just started and you were within the 7 day free trial up to yesterday, but it has now expired. The assignments are behind the paywall, but they give you 7 days free at the very beginning to try them out.”