Godking,
Your answer is in your post....

You need another cougar-claw arrowheads to continue crafting some unfinished cougar-claw arrows. You believe you can assemble the two ingredients together once you acquire them.

[Ingredients can be added by using ASSEMBLE Ingredient1 WITH Ingredient2]

Right there.. in the brackets [Ingredients....]

I looked but you didnt seem to assemble 1 with 2, but I may have missed it.

Good luck & yes, the books are missing & with scroll its possible to miss that assemble line. And I'm no current day fletcher, I did it a very long time ago.