Thank you for using Apple Support Communities. It's my understanding that you're unable to take a screenshot on your device. I'm happy to help. First, use the steps in the link below to make sure you've followed the correct steps to take a screenshot on your device:
How to take a screenshot on your iPhone, iPad, and iPod touch
If you're trying the appropriate steps and the issue persists, restart your device. Restarting can resolve many unexpected behaviors.
Restart your iPhone, iPad, or iPod touch
Let us know if you're still unable to take screenshots. Please include any other troubleshooting steps you may have tried.
Have a wonderful day!