Requesting a Refund
We're sorry your Mac isn't compatible with Knock, but unfortunately we are unable to directly refund your app store purchase. Here's how to request a refund with Apple:
Step one
Click this link to head to Apple's Report a Problem website. Once there, sign in with the Apple ID you used to purchase Knock.
Step two
Once you've signed in, click the Apps tab at the top of the page.
Then find Knock in the list of apps that appears,
and click the Report a Problem button in its row.
Step Three
Click the "Choose Problem" dropdown that appears. Select "Problem is not listed here." 
Step Four
In the textbox that appears, type:
"This app only works when paired with a Bluetooth Low Energy Mac.
My Mac does not support Bluetooth Low Energy."
Step Five
Click the Submit button. You're all set! 