This achievement can be tedious as it's RNG based. The code for the safe is randomly generated on each playthrough. To get this achievement, the generated code must be made up of all the same numbers, whether that be, for example, 1-1-11, 22-22, etc. As long as all four input numbers are the same.
Note: The achievement won't pop right away. It pops after you turn the postcard into the toy boat.