How can i defeated a monk in flotsam at dice poker.If i win he didn"t give me anything.

This was an "Easter egg" / GOG advertisement hidden in the game. If you beat him with a certain combination, it would unlock a link to some offers for GOG games: https://witcher.fandom.com/wiki/Penitent_Monk

This promotion no longer exists so the monk now always says that you're not lucky enough. You can still play poker dice with him if you really like the mini-game. Since he is not related to any quests, you can also happily ignore him and won't miss anything.