The book off the PTR vendor to learn every legendary power isn’t working for the Night Fae Druid Celestial Spirits power. It teaches every other power, but Celestial Spirits remains grey. Didn’t get a response on the PTR forum if this was intended or a bug; have tried using the book on multiple different characters, multiple covenants, older character copies, and progressing the new campaign questlines but still no luck.
PTR has their own segment of the forums, you have to post there.
https://us.forums.blizzard.com/en/wow/t/how-to-submit-bug-reports-in-the-chains-of-domination-ptr/936771
And you shouldn’t expect one. Sorry to be so blunt but that is just how it is when providing feedback or bug reports, you aren’t guaranteed an answer.