You will have to beat Rose in Rock,Paper, scissors. Her gallery in that game tell you what password her safe has for current game.
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I did everything as it was said, first I defeated her and got the code, then I went to the door, entered the code and it doesn't work. At the same time, I know that the code is constantly changing with each launch of the game, but no matter how many times I would not try it, it still does not open this door for me.