how do i make a camera with a screen gui button?

ViewportFrames are not out yet in prod of Client, only stuck for Studio with no ETA on release, so I wouldn't count on them. I think what you mean is moving the camera to a new location when you click.

You can do this in your localscript in your gui Button

local button = script.Parent
local player = game.Players.LocalPlayer
local camera = game.Workspace.CurrentCamera

button.MouseButton1Click:Connect(function()
    camera.CameraType = "Scriptable"
    camera.CFrame = CFrame.new(0,0,0) * CFrame.new()-- Change these co-ordinates to the position you want the camera to be at.

    camera.Focus=CFrame.new(10,10,10) -- Set this to the position where you want the camera to look at from the original position. So, it would look in the direction from 0,0,0 to 10,0,10.

end)