unable to take screenshot

Why can’t you take a screen shot? Here are instructions for all iPhone models, as you didn’t say what iPhone you have→Take a screenshot on your iPhone - Apple Support

what model phone , and describe the EXACT steps and what happens when you do. As it works on over a billion phones most likely it is something you aren’t doing right.

There’s a small but real possibility that if this is a company-issued phone or you have a company email account on your phone your company doesn’t want you doing screen shots to protect company private data.

I Use Back Tap on your iPhone to set up Screenshots.