If you switch from the standard WordPress editor to Avada Builder and see the popup as shown in the image below, it is due to one of the following reasons:
Invalid content editing in default editor
Invalid structures of shortcodes (elements) were created with the shortcode generator; or HTML tags have been added to the page contents without using Avada Builder. Check through the HTML for stray or incomplete tags.
Third-party plugins altering the page contents
A plugin has altered your page content by adding extra content without Avada Builder elements. Try deactivating your plugins (except Avada Builder and Avada Core) to find the culprit.
Content contains outdated shortcodes
This will happen, in particular, if you have updated from an older version, or copied code from a pre 5.0 version of Avada.