What is the issue?
PDF/UA > Basic requirements > Natural language > Natural language of text object > Natural language of text object cannot be determined
What does it mean?
There is no document language determined.
How to fix it?
Use the tab "Settings" in axesPDF QuickFix and enter the language code into the field "Language": for example "en-gb" for British English, "en-us" for US Englisch or "de" for German.