I'm assuming you are trying to set up an ebook, not a paperback, Is that correct? Unfortunately a file that looks perfect in Word or as a pdf may not work at all as an ebook. The reason is that ebooks don't have set page sizes, it will vary depending on the reading device and the settings the reader has chosen.
You can't use custom fonts in KDP ebooks. If the system doesn't recognise your font, that will get your file rejected. Basic fonts like Times New Roman, Georgia, or Arial are safest. You
can use custom fonts for KDPP paperbacks on Amazon, by creating your book file as a pdf with the fonts embedded. If fonts aren't embedded, that will get a paperback file rejected as the system won't be able to render the font.
Other possible ebook issues?
1. What format are you uploading? KDP now works best if you upload an epub file for ebooks. It will manage a docx file, but the file size will be larger which can cause problems. It absolutely won't use a doc file, either, even though it says it will. Also pdf uploads won't work for ebooks, but are the preferred upload format for paperbacks. KDPP won't accept all pdf formats though. some of the newer pdf formats get rejected.
2. If you're pricing your ebook in the 35% royalty band, your converted book file size in Amazon must be less than 3Mb, or it will be rejected. The solution if that's the issue is to change the pricing to between $2.99 and $9.99. Then there's no limit on file size because they take the download cost from your royalties. That can cost you quite a chunk of possible earnings, if the file is very large, however.
They have a downloadable tool for kids' book set up that may help:
https://kdp.amazon.com/en_US/help/topic/G201562880 If will mean needing to layout your book interior again, but
should fix any issue blocking the upload by ensuring your book is laid out in a way Amazon can work with.
If you want more specific advice, knowing if this is ebook or paperback and screenshots showing what the error message says when the file is rejected would be helpful.