What is the issue?
PDF/UA > Logical Structure > Structure Elements > Headings > First heading level > First heading is not on the first level
What does it mean?
In every document, the first heading has to be an H1. The first heading in your PDF, however, is not an H1.
How to fix it?
In the Logical Structure Tree task pane, change the first occurrence of a heading tag to "H1". Be careful that there is no jump in the heading levels. You must not miss a level; for example, an H1 must not follow an H3.