What is the issue?
PDF/UA > Logical Structure > Structure Tree > Note structure elements > Possibly inappropriate use of a Note structure element
What does it mean?
Some note tags in your document are not used formally correctly. In most cases it is due to the placement attribut. Innately the note tag is an inline level element. If it is used as a block level element, the attribute "Placement: Block" is required. Otherwise some assistive technologies are not able to present it correctly.
How to fix it?
Use the the tool Fix Placement in axesPDF QuickFix.