No output enable on Bidirectional pins

Non-Z driver simply means that a signal is send to the output. In VHDL or Verilog designs, driving an output to 'Z' state is equivalent to disabling the output driver. It's the ususal technique to infer a tristate driver in HDL.

I understand that you have an tristate driver connected to the pin, either in schematic entry or as explicite instantation of a tristate primitive in HDL. In this case, the error means that the output enable of the tristate driver is never activated cause no respective logic exists in your design. This may be, cause your design is yet incomplete or the logic doesn't correspondend to the intended function.