This portion of the Luigi’s Mansion 3 guide contains a walkthrough for your first official objectives. After installing the elevator tracker, you’ll need to practice contacting Professor E.Gadd by opening up your menu and connecting to the Professor E.Gadd Hotline. Once that’s done, you’ll have your main objective: recover E. Gadd’s Briefcase.
These objectives will take you through the Basement (B!), Grand Lobby (Floor 1), and part of the Mezzanine (Floor 2). Near the end of this mission, you’ll have to defeat the Maid Boss who has swallowed Professor E.Gadd’s briefcase. After returning to the lab, Professor E.Gadd will give you the gift (and curse) of being able to summon Gooigi.
Source: IGN.com Luigi's Mansion 3 Guide: Recover E.Gadd Briefcase Walkthrough