Updates to the KDP Amazon Publishing service, make some LaTeX templates difficult to be accepted. Avoiding the error for previewing, seems to entail two key ingredients for LaTeX users:

1) create a PDF  with the settings “PDF/X-1a”, which can be achieved via

\usepackage[x-1a]{pdfx} 

2) flatten the PDF using Adobe Acrobat, Other Tools, Print Production -> Flatten.