What should I do if the option to 'Allow your Apple Watch to unlock your Mac' is absent from my options?

I have an Apple Watch (Series 1) and a 13" Retina Macbook Pro (2015). My watch, iPhone 6 plus and Macbook are all using the latest OS. Sadly, I can't unlock my macbook with my watch since the option to 'Allow you Apple Watch to unlock your Mac' is not available in my system preferences > security. I'd like to know if there is any way around this?