Screenshot not working in iPhone 12 pro

Try a Forced Restart on your phone:

• Press and quickly release volume up button
• Then immediately and quickly release volume down button
• Then immediately press and keep holding the side power button (keep holding past the swipe to power off) until the screen goes black and until the Apple Logo appears, then let go
• Sign back in and then try to take a screen shot

The easiest way is to setup a double or a triple tap on the back of iPhone to make a screenshot.

Here's how => Settings > Accessibility > Touch > Back Tap > select Screenshot

How are you trying to take a screen shot? The correct method is to press the volume up and power button at the same time.

Try Force restart iPhone - Apple Support