Some options:

Hey, you're not on the list, you can't vote.

Hey, you're not on the list, use the computer in the corner to enter your details and see if you're eligible.

Hey, you're not on the list, here is a provisional ballot, we will verify your info and if it clears, your vote will be counted.

You should not get 5 years in prison because the election board is incapable of securing their processes.