When you upload a Word document to Leanpub, it needs to be in the .docx file format.
If your book is in the .doc file format, .you should be able to use Word to convert your document to .docx by using the "Save as" feature in Word, and then choosing .docx from the options presented to you.
To do this, open your file in Word and select
File in the menu at the top of your screen, and then select
Save As... . Then you will see an option like
File Format with a drop-down from which you can choose the .docx format. Finally, click