How to take screenshot on Huawei P9?

Welcome to the forums. Have you already tried pressing simultaneously the volume down and power keys for a second?
 
Welcome to the forums!

I used the Huawei P9 before. Pressing volume down and power at the same time should take a screenshot. On a side note, there should be a Knuckle command for it to do so.
 
Try double knock on the screen. The settings are found under motion control. That will do the trick.