Are you sure you're trying to go in at the right time? It says there that you enter on the First or Second night between 6 and 12 P.M. and that's when you can do it. If you're just locked out, it could be part of the game, but if you already checked the times and everything, I need to suggest that it's a glitch.
This this happened to me too, it seems to be a glitch or programming error. I stood outside the Post Office almost 48 hours and I was never able to get in. I played the song of Time and tried again and it worked perfectly for me. I hope this helps.