The Sidecar feature is intended to extend the Mac display to iPad - not iPad to Mac.
If you need to display your iPad screen on an external TV or monitor, there are a number of methods to explore:
1) If you have an AppleTV box, or your TV supports AirPlay2, you can simply use the iPad AirPlay feature to “mirror” your iPad display on your TV/Monitor.
2) You can use a suitable HDMI Adapter and cable to make a wired connection to a TV/Monitor. Assuming that your iPad has a Lightning port, you will require an Apple Lightning Digital AV Adapter:
https://store.apple.com/uk/xc/product/MD826ZM/A
3) If you wish to display your iPad screen on a computer, you will need a third-party software application - for example, Reflector3:
https://www.airsquirrels.com/reflector
A further possibility is to install AirPlay-enabled software on your computer - and again use AirPlay from your iPad. One such example is Microsoft AirServer:
https://www.airserver.com/PC
