Not all USB-C ports that you find on an external monitor are designed to accept video input. You will need to check the monitor specifications to see if the USB-C port supports DisplayPort over USB-C (a.k.a. DP Alt Mode) or Thunderbolt3.
Unless either of these standards are supported, you’ll not be able to make a direct connection using a USB-C cable.
Perhaps your monitor supports HDMI or VGA video input. For these, you will require both the appropriate adapter and cable. For Adapters:
- USB-C Digital AV Multiport Adapter (HDMI):
https://store.apple.com/uk/xc/product/MUF82ZM/A
- USB-C VGA Multiport Adapter:
https://store.apple.com/uk/xc/product/MJ1L2ZM/A