Hi Ravji,
Hope you are well!
If your event is a private event (Only those on invitation list can register), then please make sure that the invitees are registering with the email address that is added for them within the invitation list. If they'll try to register with any other email address, they'll get an error.
Could you share the name of the event so I can check the set up? Also, could you share a few examples of the invitees who received the error and the email address they used to register for the event?
Additionally, you can refer to the following article for your reference :
How do I fix the "Registration for this event is not available" error message? - LINK : : https://support.cvent.com/s/communityarticle/How-do-I-fix-the-Registration-for-this-event-is-not-available-error-message?
I hope this helps.
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Raashi Semwal Khati
Quality lead - Client Services
Cvent
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