Hi,
Hope you are well!
As Tori correctly mentioned, you need to add the 'registration navigation' widget on the page the system directs you to as without this widget, the invitees will not be able to move forward in the registration.

Once you have added this widget, you should be able to save and publish your website. Also, for your future reference, when you are done making changes to your website, you can click on the (...) next to 'Device Preview' on top right and click on 'validate website' option to check if you are missing on adding any required widget before you publish the site designer.
You can also refer to the following article for your reference :
Using the Site Designer = LINK : : https://support.cvent.com/s/communityarticle/Using-the-Site-Designer
I hope this helps. Please let us know if you have any further questions.
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Raashi Semwal Khati
Senior Quality Analyst
Cvent
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