The answer to 'why is my AC not heating' is that your air conditioning unit is not functioning in heating mode. This could be due to a few common issues, such as a thermostat malfunction, a problem with the heating elements or compressor, or a lack of refrigerant. Your AC unit may also be set to cooling mode instead of heating mode.
If your AC is not providing heat as expected, first check that the thermostat is set to 'heat' and the desired temperature is higher than the current room temperature. You can also try replacing the air filter and ensuring the outdoor unit is free of debris. However, if these basic troubleshooting steps don't resolve the issue, it's best to call in an HVAC professional to diagnose and repair the problem, as improper heating can be a safety hazard.