I’m returning to playing some alts and trying to work on them, but the optional reagent slot requires helping the runecarver more? I’ve done the crafting material quest in Zereth Mortis and I’m honored on this character specifically, but I’ve found no solution by googling this.
Edit:
It seems you have to do a bunch of Korthia to use this. Through chapter 4. That’s… pretty bad design for a catch up method.
On another note, I tried to get an alt through Korthia and they’re stuck on
Unable to pick up the quest.
Not sure what to do here, but locking current patch reagents and materials for a two patch old questline and god knows what else, it feels terrible. Good luck to anyone who’s trying to raise their professions up for the first time now in SL.
A GM sent me the link to this wowhead post that I’d already read with a “Glad I could clear this up for you and I wish you all the best!” when this did absolutely nothing to help.
If you just want to craft 291 legendaries, you can ignore the covenant campaign and Korthia and just complete up through the Crown of Wills chapter in Zereth Mortis to unlock the vestige and the reagent slot. You would still have to go back and complete those if you’re interested in unlocking the 262 vestige, but that’s all. I did this on my jewelcrafting Paladin who hadn’t done the covenant campaign, and only campaign skips.
I’d done that semi recently and had to complete Korthia 
You may want to report a bug or try a UI reset to rule out interface issues if that happens again. I had a few characters that did the above and unlocked that reagent slot with only the relevant ZM campaign progress.