Where is it? I've done all prerequisites but I can't find the challenges 

The entrance is unlocked during the baking event with Dea and Mai. It's the sparkling fireplace in Nina's bakery.

good morning i past the zone with orc i open the stone gate with the code i found in the wood chest and after i can't pass the riddle with the button i thinks it's because i'm not very good with english(i'm french ) can you help please

Maybe this helps: https://renryuu.miraheze.org/wiki/FAQ#2021_Christmas_special
"Fat Orc button dungeon: You need to press 7 Red, 8 Blue, 9 Yellow and 10 Green buttons, and use the switches of each color once you have the exact right amount of that color. (The amount of buttons for other colors don't matter for the switches.)"