Problem

You don't have a webcam on your computer.

Cause

Many older computers and some newer desktop computers do not have a built-in webcam.

Procure an external USB webcam and plug it into your computer.

If you have a smartphone or tablet, download the Zoom mobile app from the App Store (iPhone or iPad) or Google Play (Android phone or tablet).
Connect to your Zoom meeting from your mobile device.
Use the camera on your mobile device.