Full Platemail is uncraftable.
I’ve tried crafting a Full Platemail, having (more than) four steel bars, one thread, and one bundle of leather. My blacksmith stands there, unable to do anything else. I’ve removed the Full Platemail from her queue and put another item in (a bronze shield), and she was able to craft it without a problem, so it is definitely the Full Platemail recipe.
Steps to reproduce:
Attempt to craft a Full Platemail.
This issue appears similar to this one: [Res] Cannot craft Thick Leather Vest
EDIT: Nevermind, non-issue. I was one steel bar short.