I've seen other project can easily control button via 'Process value'

but I can't find it, can someone guide me

A button doesn't have a process value.

Actual it does. Hmmm... Let me see.

Click on the button to make sure you give it focus after you open the properties pane.

Edit: Which version of TIA and WinCC are you using? I can't seem to replicate your screen view.

Here are button properties

TIA V15.1

WinCC Pro V15.1

Additional in Advanced ver still have Process value

am I confuse or I do anything wrong

I have a button process value in the interface properties but not entirely sure what I can do with it as it doesn't really do much.

What are you wanting to achieve? You maybe trying to do something that can be done another way.