What is the issue?
PDF/UA > Logical Structure > Structure Tree > Document structure elements > Document structure element found which is not a root element
What does it mean?
There are "Document" structure elements in your document that are not used semantically correctly.
How to fix it?
Double-click on the error in axesPDF. The related tag will be highlighted in the task pane Logical Structure Tree. Rename it by clicking on the tag and overwrite its name with a semantically correct tag name - for example "Part".