[UPDATE September 2021: The Kindle app now works on Big Sur!]
This is a know issue the Kindle app on Mac (here's an article about it if you're interested in learning more).
The good news is you can still get Leanpub ebooks onto Kindle for Mac, you just have to use the Send to Kindle app to do it.
For more comprehensive information about getting a Leanpub ebook onto Kindle, please see this article from our Help Center.
It has also been reported to us that Send to Kindle doesn't work for Kindle on macOS Big Sur; we're investigating whether or not this is true, but of course whether it's true or not, there's nothing we can do but wait for a fix from the people at Kindle.