LibreOffice is 100% free, and is basically a simple version of Word. I use it for everything I write.
From what you're saying, it seems like you're having the same problem I did: there is superfluous code in your writing that is bugging up Booksie's editor. Here are the steps you need to take to fix it:
1. Download LibreOffice and install it. Go to your story/chapter and go to the Edit Content screen (where you actually typed in your story). Copy the entire text of your story.
2. https://www.striphtml.com/ Paste your content into here, click the button, and then copy the text it gives you. This strips all formatting from your document, including the bad code.
3. Open LibreOffice, paste in your content, and save it (you should always back up to your computer AND to a cloud/external hard drive). Apply any formatting you need, and save it again.
4. Go back to the Booksie editor, copy the entire text of your story and delete it.
5. Look above the box you just emptied. You should see three lines of icons. The third one on the first row is a W in a box on top of a clipboard. Click it. A box will pop up, paste your work into it, and click Ok.
6. Scroll to the bottom, click the checkbox, and hit Save and View Chapter Content.
7. View your chapter content to be sure, but everything should be groovy now.