I'm struggling to get the code working, I had to start over and over again but no success. I was hoping if anyone could help me with it. Plan was to push 4 buttons (out of 11 buttons) as a combination to either lock it or unlock it. I also have reset button so I can reset the combination so I can do it all over again.
This is my schematic for school project:
I count only 10 buttons for the combination entry.
If you've been struggling, no doubt you have some code to show which you should post here. Have you got as far as writing some code to read even one button ?
Yes, 10 buttons are for the combination entry. My mistake. About the code not really, I'm still learning it.
I tried to start by collecting some "examples" from the internet, I wanted to combine them into one but it didn't work out. In other words I don't really know how to write code. I'm more comfortable around making printed circuit board ( which I already made ).
Start like this to read one button and gradually progress from there.
The labels on the switches on your schematic does not correspond with the pins these are connected to on the processor chip.
Switch 1 is labeled MCU_pin11 but is connected to pin 28 which is known as Arduino pin A5 which you read by digitalRead( A5 ).
Also your schematic shows AREF tied to +5V. That is unusual. See here.
Okay, I will fix this labels on the switches. About schematic, my mentor said that it's alright, now I'm hoping he didn't make a mistake or he didn't saw the mistake. I will ask him again.
For the code, I will start as you told me. Start with button and to progress from there. Thank you. 
Here is a simple keypad door lock that works very well.