PAYDAY 2 is one of those games where cooperation is absolute crucial to your success. In order to fully work with others, you need to talk to them. When it comes to PAYDAY 2 on Switch, there's no built-in voice chat option for players to use. Why didn't Starbreeze use Nintendo's Switch Online App to handle voice chat? Seems like they want to, but weren't allowed to yet.
“We are waiting to be able to participate in Nintendo’s VOIP app solution. As soon as we are given the go-ahead, this is definitely on our list to look into to update.”
If you're playing PAYDAY 2 on Switch, it looks like you'll have to use your own solution for voice chat. It seems that the Discord or Skype mobile apps are the best way to get this done.