The answer to 'why is my air conditioner not heating' is that your air conditioner is not designed to heat your home. Air conditioners are cooling systems that remove heat from the indoor air and release it outside. If you need heating, you likely need a separate heating system such as a furnace, heat pump, or space heater.
Some common reasons an air conditioner may not be heating properly include issues with the thermostat, refrigerant leaks, or problems with the compressor or other components. If you are experiencing issues with your heating system, it's best to call a professional HVAC technician to diagnose and repair the problem.