What is the issue?
PDF/UA > Basic requirements > Natural language > Natural language of actual text > Natural language of actual text cannot be determined
What does it mean?
There is no document language determined. As a consequence: There is no language determined for the actual text.
How to fix it?
Use the task pane "Settings" in axesPDF QuickFix and enter the language code into the text box "Language": for example "en-GB" for British English, "en-US" for US English or "de-DE" for German.