The Develop module is disabled when you have the Creative Cloud version of Lightroom installed, and are not signed in with an active subscription.
If you are not a subscriber to Creative Cloud, but purchased the perpetual license, you'll need to do two things:
1. Uninstall the CC version of Lightroom completely;
2. Install the stand-alone version of Lightroom 6 (helpful links via the great folks at ProDesignTools):
Instructions for installing the stand-alone version: New Adobe Lightroom 6 (CC) Direct Download Links – Free Trials | ProDesignTools
Visit this link and sign in with your Adobe ID before clicking the links below: http://prodesigntools.com/go/tryphotoshop_elements
Then click here for the Windows version of LR6: http://prodesigntools.com/trials3/AdobeProducts/LTRM/6/win64/Lightroom_6_LS11.exe
Or click here for the Mac version of LR6: http://prodesigntools.com/trials3/AdobeProducts/LTRM/6/osx10/Lightroom_6_LS11.dmg
If you don't uninstall the CC version, you may continue to have difficulty.