Like a good many of things in Wix, certain features won’t work in preview mode. You will need to be testing the site on a fully published website and logging in as a site member etc.
“The functionality for logging users in and out of your site only works fully when viewing your published site. You will not be able to fully test your member pages in preview mode.”
“When you preview the page, you are the logged-in user, so you will see your own content. If you have don’t have any items in the collection, your page will not show any content when you preview it.”