What is the issue?
PDF/UA > Logical Structure > Structure Elements > Annotations > Nesting of Link annotations inside Link structure elements > Link annotation is not nested inside a Link structure element
What does it mean?
There is a link annotation in the PDF but that annotation is not nested inside a link tag.
The logical structure tree may appear as follows:
How to fix it?
The link annotation must be located within a link structure element. In the example above, the simplest solution would be to rename the Span structure element accordingly with axesPDF.
Note: You can also use axesPDF to create a complete new link structure element (first right-click in the logical structure tree and then click the Insert new tag or Enclose elements option).
However, if the link annotation does not exist in the logical structure tree (i.e. it is not referenced in the structure tree), you must search for it (e.g. using Adobe Acrobat) and create the object reference in the structure tree.
If the non-referenced link annotation is present by mistake, you can remove it using the Delete Non-Referenced Link Annotations tool.