Please check the version of Lightroom that you are using.
Help -> About Lightroom (Windows) OR Lightroom -> About Lightroom(MAC)
Also dehaze is available only with creative cloud subscription, do you have the creative cloud subscription ?
it was on there when i first downloaded lightroom CC . I have paid for a subscription
Check that you're using Process Version (PV) 2012.
Check this article to check and update the process version : Updating the Process Version in Develop presets from Lightroom 3 to Lightroom 4 and later.
Also make sure that you are signed in to creative cloud app and no update is pending for lightroom.