4.1.1 Parsing (Level A) issues when exporting

Hello, We have just recently started our accessibility journey, so we are working on going back through some previous courses so that we can make them more accessible. We have joined with a vendor to audit the first course and one of the comments was "The HTML code of the website does not validate against the HTML 5 specification. The source code of the page presents 147 errors and 3 warnings when checked with the Nu HTML Checker. In particular, several instances are found where elements not nested properly. As a result, the HTML validator generated errors".  When I went back to them, they stated the issues was on every page. Now according to Storyline's VPAT - WCAG section 4.1.1 (https://articulate.com/support/article/Storyline-360-Accessibility-Conformance-Report-VPAT) The html output is "well-formed".  Has anyone else run into this or had this issue or would know how best to fix this?