Complete manual fix
Leave the required back-cover barcode area clear or follow KDP's barcode workflow. Do not place essential text or imagery beneath it. Download a new retailer template after the interior is final. Compare the PDF page dimensions, bleed, folds, safe areas, barcode zone, and spine text against that template before upload.
Coverfairy shortcut
Enter the final trim, binding, paper, and page count. Coverfairy can rebuild the KDP wrap, and the $19 Print option is covered by the technical spec guarantee.
Keep the evidence
Save the retailer's written notice, the unchanged exported file, the selected print settings, and the template used for comparison. Those records make a technical support case reproducible.