Unable to take screenshot

That’s because you are holding the buttons. Just press and release quickly. You can also enable a back tap on the phone to take screen shots. Go to Settings/Accessibility/Touch/Back Tap and you can program 2 and 3 taps to do different things, in addition to screen shots.

1. Go to Settings  > Accessibility > Touch > Back Tap.
2. Choose Double Tap or Triple Tap, then choose an action. => Screenshot
3. Double-tap or triple-tap on the back of iPhone to trigger the action you set.

Note you need to tap quickly.

Please kindly reset your iPhone settings by using this guidelines from Apple

Return iPhone settings to their defaults - Apple Support

Did you follow these instructions→Take a screenshot on your iPhone - Apple Support