So I had two Pokemon in my party, and when I tried to move one of them to the pc, it said it was my last Pokemon! It was only doing this for one Pokemon, it let me put the other one in the pc.
The Pokemon it didn't let me move to the pc was a dratini, the other party Pokemon was a riolu. This was in platnium, I have beat 7 gyms, and I need to beat cyrus.
There are a couple scenarios where this could have happened.
The last Pokemon you want to have out (Riolu) is fainted, which is not allowed, therefore not letting you put Dratini into the PC.
The game is glitched and isn't letting you put Dratini back into the PC.