Something has not been set up correctly and the registration has been closed. Check out these 7 settings:
Registration settings
The registration is closed on the date in Event details - Basic.
The Register page is not active in the Website Module - Pages (the page must be checked).
The registration is closed due to the maximum number of participants in Event details - Basic.
Ticket settings
There is no ticket available in the Ticket Module.
The 'start and end date ticket issuance' of the entrance ticket is on the wrong date.
Are lists linked to entrance tickets?
The invitee receiving this notification has not been added to an invitee list.
The invitee who receives this notification is in an invitee list that is not linked to a ticket.