Extensions menu option is greyed out

Do you have any Extensions loaded/installed?

Click on Find Extensions on Exchange

Make sure Photoshop is checked on the left of the page, and look at the free Extensions.

This has long been a favourite of mine.  It allows you make targeted masks according to tonal range i.e. highlights, midtones shadows. and use those masks in Adustment layers (Curves, Levels).  It's superceded to some extent nowadays because we can do a better job with Camera RAW, but it will let you test that greyed out menu.