Didn't Receive Rank 3 Recipe for Mech-Dowel's "Big Mech"

The World Quest Work Order: Mech-Dowel's "Big Mech" will only reward the Rank 3 recipe if your character has already trained Rank 2. Both Rank 1 and Rank 2 can be trained with the cooking trainer at your faction's Nazjatar hub: Rolm for Horde or Instructor Ulooaka for Alliance.

If the World Quest was completed before training Rank 2, you should still receive another reward such as Azerite, but you will need to complete the World Quest again next time it appears in order to receive the Rank 3 recipe.