Messi is not comfortable at PSG because most PSG fans don't like him. He was the winner of the World Cup title when Argentina beat France in the final, so of course some PSG fans who incidentally are French citizens are not happy with Messi's presence. Since Messi defended PSG, I don't think he gets good respect from some of the fans, obviously it's bad for the psychology of the player.
Kylian Mbappe left because of his desire and ambition to achieve individual achievements with other teams. He's having a hard time proving himself better at PSG because his team can't win the Champions League title, but that's possible if Mbappe plays for a team like Real Madrid or another top team. The situation is not conducive, but it's not PSG who want it but it's because of the player himself.
In fact, they didn't like Messi because they were expecting to see a better performance from him, and for any loss in PSG and anytime when the team had a bad performance they were blaming Messi and some other players, while players like Messi needed some more respect in the team he plays there. That's why he couldn't have enough focus there.