It's my bad with the hint it should stop saying you need Serena after you've done her quest.
It talks about Serena because her quest is a fast way to complete the Battledex but you also need to find some of the random trainers scattered around.
Calm down Faust, calm down
you don't have to be like that, man
everyone makes mistakes, and it's normal
whether it's a mistake or not, you'll continue to be an incredible person, so you don't have to be like this, sometimes we can always make one mistake or another, that's normal, it's how a person lives and learns, so there's no need for that, you're fantastic and your projects are incredible, so stay calm, man