Hi,
While registring the event getting the following error. Kindly let us know what is causing the issue.
Error:
An Error has occurred:
Your form data could not be applied to any valid registrations.
This can sometimes happen if too much has been taken to complete the registration process.
Please return to the Event List and reselect your tickets. If the problem continues, please contact the site administrator.
Just noting that the issue for @Kandil was a conflict with out MailChimp add-on and BackWPUp.
If you have MailChimp enabled on the site check the event editor and see if shows an error in the Mailchimp metabox there, if so, deactivate MailChimp and test another registration to see if works then.