Complete manual fix
Download a new official IngramSpark cover template for the exact title setup and use its spine width. Do not substitute a KDP paper formula. 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
Use the exact spine from IngramSpark's current template. The cover studio exports a matching paperback PDF after you enter that measurement; the official generated template remains the authority for the final IngramSpark file.
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.