OK, create a variable in the OnVisible property of the screen:
UpdateContext({varButtonVisible: true}) // Turns the button visible by default
Use this new variable in the Visible property of the button:

Then edit the OnChange property of the combo box:
If(
First(ComboBox1.SelectedItems).DisplayName="jack",
UpdateContext({varButtonVisible: true}),
UpdateContext({varButtonVisible: false})
)
This will turn the button on and off according to the selection of "jack". Hope this helps!