The usual causes are text or page numbers outside the margins, a cover that doesn't match the spine width for your page count, fonts that aren't embedded, low-resolution images, and a title or author name that doesn't match between the cover and the listing.
Fix order
Check the trim size and page count first; many other errors follow from them. Then re-make the cover at the final spine width.
In Author Magic
The KDP pack is checked against the trim, margins, spine and fonts before you upload.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.