If you correctly copied the error text then this means that the pin in question is defined as power input. (Why would you do that for the base contact of a bipolar transistor. You might want to look at Electrical type of schematic symbol pins (KiCad 4 and KiCad 5))
A power input pin needs a power output pin on the same net. If you come to the conclusion that power input is the right type for that pin then look at ErrType(3): Pin connected to some others pins but no pin to drive it
And generally: Why would you use a bipolar transistor in a switching application? I would have expected a power mosfet or an IGPT. (Or is your symbol simply wrong?)