What is the issue?
PDF/UA > Logical Structure > Structure Tree > Part structure element > Part structure element used as root element
What does it mean?
There are Part structure elements in your document that are not used semantically correctly.
How to fix it?
Double-click on the error. 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" in a PDF tag editor and nest all other tags in it.