What is the issue?
PDF/UA > Logical Structure > Structure Tree > Art structure element > Art structure element used as root element
What does it mean?
There are Article 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 root tag name - for example "Document". Or create a new root tag "Document" with axesPDF and nest all other tags in it.