There’s been many times in BGS where PW:L is greyed out and unable to be used. The button still lights up due to the person being able to benefit from it. I also experienced Mind Bender also being greyed out despite being talented for it.
You might want to uninstall add-ons (temporarily) and try a UI update and see if that’s the issue, if you haven’t already:
I’m having the same issue in PvE content. It is unusable and returns a spell is not learned error. I thought it was fixed the other day when I logged back in and ot worked but later in the day I was unable to use it again.
My issue seemed to be associated with a macro for the spell. The other day my macro was giving the error but I could still target the player and cast it from the spellbook. The macro resulted in spell not learned.
The macro was /cast Power Word: Life, nothing complicated.
The issue so far has been intermittant over two days and I cannot identify a way to recreate the issue. Simply playing throughout the day and I would realize in a dungeon that it stopped working.