We check if the data item matches a live URL using the HTTP Status Codes. If you have a 301 redirect error, you need update to the new page. A 404 is missing you will need to update the data item to wherever the data item exists on the web.
Items using the default http://schemaapp.com/db/# URI will show as 'undef' - click on the item to go to the Edit page and use the Action/ChangeURI to change this to a URL within your own domain. Items that have no page should use the '#type' notation, eg https://www.mydomain.com/#image_1 (NOTE: the item URI must be unique within your project)