To complete Mission 15, like in Mission 9, all you have to do is upload a creation to the challenge 'Build Nexus Tower!'. When the creation is approved, you send 200 minifigs through to LU. You only send minifigs with your first approved creation; subsequent creations do not send minifigs.
Mission story
The Nexus Force needed a headquarters to organize the countless minifigs answering the call from across the Universe. Doc Overbuild gathered his colleges from the Build Alliance to design a tower that would not only protect the last remaining piece of pure imagination, but stand as a beacon of unity and hope for the future of imagination. He called this new structure Nexus Tower
B.R.I.C.K. Tip
Another building challenge where your imagination is really put to the test. Think hard and good luck!