The moonlight mode is a mode in PUBG mobile like the night mode in Erangel that happens only in Vikendi. And same as the night mode it only appears randomly and cannot be controlled. If you really want to play in Moonlight mode you will have to play solo and in the spawn, island checks the bottom right of the screen to know whether it is a day or moonlight mode. If not, exit the match and try over and over until you have it. This should only be done in Solo mode because in Squad mode it will get you banned from team matches.