What operating system are you using?
You updated to photoshop cc 2018 or installed the just released update to photoshop cc 2018?
This is a feature that has certainly caused me much frustration and unfortunately Adobe has done a poor job documenting the "copy layer style" feature in my opinion...
Basically with Photoshop CC 2018, the "copy layer style" does not copy text formats, but instead copies any EFFECTS that were applied (such as drop shadow). If there are no effects applied to the layer, the feature is greyed out. It would seem to me that a more intuitive labeling of that feature would be "copy layer effects".
But possibly what you really wanted was the ability to copy text "formats" like you can do in MS Word with the copy formatting button? If so, I am not aware of a way to do that (if some else knows - please comment!).
-- Use case:
As an example, lets say you have multiple headlines on a page. You make tweaks to the style of one headline, leaving the others as-is to be able to compare the changes. Once you have modifications that you prefer, then you would like to simply copy the newly modified "text styles" to other layers. Something like "copy CSS" and then "paste CSS", but I'm not seeing any way to paste CSS in the current version.
Anyone know of a way to do that, or to achieve the desire result another way?