What is the issue?
PDF/UA > Logical Structure > Structure Elements > Headings > Nesting of heading levels > Numbered heading skips one ore more heading levels
What does it mean?
The headings hierarchy in your document is formally not correct. One ore more heading levels are missing.
How to fix it?
Double-click on the error so that the related heading and heading tag are highlighted. Change the name of the tags by using the command "Rename Tags" or edit the tag names manually in the Panel "Logical Structure Tree".