Hello, i have made a voting system using digital logic, but i have a one problem. How can i put protection that one voter can vote only one time and only yes or no?

 

You have 6 pairs of switches. Why? There is nothing to distinguish one switch from another.

The switches aren't debounced, so pressing one of them will probably register many votes.

If you are going to ensure that a given voter can only vote once, then you have to have a means of locking out further voting once the first vote is cast and then have a mechanism for an election official to re-enable the inputs after they have verified the identity and eligibility of the next voter.

Now, are we to somehow guess that the purpose of the twelve switches is that your problem stipulates that you have (at most) six different voters and each voter will use a different pair of switches (and it is assumed that a given voter only has access to their switch pair)? If so, then it would sure be nice to know that and not have to guess at it.

 

A latch circuit at each switch with an inhibit of all voting after one vote is cast will do what you need. I'll leave the details to the OP.