The PDF is password-protected

KDP cannot open an encrypted interior. It is rejected without the rest of the file being read.

What to do

1. Security applies even when there is no open password: a permissions restriction on printing or copying is still encryption.
2. Word, Acrobat and most export dialogs put this behind a “protect” or “restrict editing” option. Turn it off and export again.
3. Re-saving an encrypted PDF through a print-to-PDF driver usually strips it, but check the result rather than assuming.
4. The pre-flight checker reports encryption as its own error, so you can confirm the new export before uploading it.

Check for encryption — the tool in this app that settles it.

Related

• Fonts are not embedded
• Text runs into the margins or the binding
• Artwork sits under the barcode
• Page count is out of range