When someone tries to register for your event, they receive the message, "Sorry, but we cannot take your registration at this time. CLOSED/Event has Started". You wish to enable registration.
The system closes registration automatically once the Event Start Date has been reached (and when capacity has been reached). So please review the date you have entered at Events -> Settings -> Start Date/Time.
Also at Events -> Settings is the field titled Registration Status. This must be set to Open if you wish to accept registrations.