Conversations involving multiple participants, allowing collaboration and information sharing in Teams
If the New Chat button isn't showing, you may be using the newer Chat and Channels layout where the option is under New items instead.
I’d first try Ctrl + N in the Teams desktop app. Microsoft lists that as the shortcut for starting a new chat.
Also check the top of your Chat list for New items or the New message icon. In the newer combined view, Teams has moved some of these options around.
If Ctrl + N doesn't do anything either, try Teams on the web and sign in there. If New Chat appears on the web but not the desktop app, that would point more toward a Teams client/UI issue. Microsoft also recommends restarting Teams or signing out and back in when chat items aren't loading correctly.
It would also help to know whether they're using New Teams desktop, web, or Teams Free, since the interface can be different.