What i can do when i can’t decline incoming call by power button and i can not find this option?

You can still end/decline calls using the power button

In SETTINGS > Accessibility > Touch, stroll down and turn off “Prevent Lock To End Call”

That should do the work…

Usually if you hit the side button it will leave the screen and stop ringing.

You should be able to mute or block people you don’t want to talk to for some time or if you’re busy, just go into contacts and then info and it should let you silence or block people and you can change it back anytime. Hope that helps!

I can decline by one time clicking of the power button .See if that works for you