[Equipping K-Drives]

I can't equip a K-Drive, even though I bought one with platinum. I go to the arsenal, select vehicles as my mode, and it has the Archwing and the Archwing weapons, a slot for a heavy weapon, and then nothing underneath that. The K-Drive I bought is still in my inventory, and I have finished Vox Solaris, and I equipped the K-Drive launcher in my gear wheel. Is there something I'm doing wrong?