I heard your moms meowth can fight another meowth
First you must complete your game and beat "episode RR". After that, go to your house and talk to your Mom, then talk to Meowth. Here's a video: https://m.youtube.com/watch?v=EVNDkRGoSQQ Info: https://gamefaqs.gamespot.com/boards/210930-pokemon-ultra-sun/76048979 If your problem persists, comment on my answer.
I hope I helped!