What's the meaning of I've got no room?

Question

Here is the quote from the American TV drama Manifest.

The police were about to send a criminal who was dying to the hospital nearby, the conversation below took place between two policemen:

A: Where are you taking him?

B: Mercy.

B: I've got no room. We got a uni riding with him already.

A: I can follow behind. But he can't go to Mercy.

What's the meaning of I've got no room.?

Answer 1

B is telling A that there is no empty seat left in the vehicle in which B is riding and taking the prisoner to Mercy hospital. This is because already a uniformed officer (a policeman) is riding along with them.