"Those pages aren't in this PDF"

Split PDF reads the pages you type as a list: 1-3 means pages 1, 2 and 3, a comma starts a new file, and 5- means page 5 to the end. Pages past the end of the document are left out, and this message appears when none of the pages you typed exist.

What to try

Check the page count in your PDF reader, then type pages within it. To save every page as its own PDF, leave the box empty.