Not sure, that URL is definitely valid:
Are you perhaps on a network that doesn't allow the git:// protocol? Try running this from your command line:
git://
git ls-remote --tags --heads git://github.com/saralsharma/adapt-download-button.git
Thanks Matt,
It was my firewall, I have listed this and an extension "Exit button" in the Adapt Plugin browser.
Cheers,
SaralĀ