Airdrop checkbox is greyed out
I'm running the latest version of Yosemite 10.10.2 and I seem to be unable to use Airdrop. The checkbox doesn't appear in the Finder sidebar preferences and in System Preferences in the Extensions page, the Airdrop checkbox is greyed out and cannot be selected.
Any idea please?
MacBook Pro with Retina display, OS X Yosemite (10.10.2)