How do I unlock final GC rank?

[Melinia]

am exactly in the same shoes. In the seal exchange screen, there are six tabs, and i am on the third, and clearly Second flame lieutenant is not the highest rank, yet it tells me that there is no promotion.

It is the highest rank at this time. The other ranks presumably are for later plans.

2nd Lt. was also the highest rank attainable in 1.0.