I wish that was an option, but B&N doesn’t really do free full ebooks for their store titles. They’re a retailer, not a library. Their Nook apps and site are built around purchasing or borrowing through their subscription thing, Nook Plus, which costs money. Sometimes you can read a sample chapter, but that’s about it.
Your best route for a free legal read is through your local public library’s digital service, like Libby or OverDrive. You link your library card, search for 'Lord of the Flies', and borrow the ebook from there. Wait times can vary, but it’s the official way. Project Gutenberg might have it, but that’s only for older public domain works, and 'Lord of the Flies' isn’t there yet.
I tried some of those ‘free ebook’ sites once for another book, and the formatting was so broken it was unreadable. Not worth the hassle, honestly. The library app is way more reliable.