Dropdown dependency

[Nofal]

I have a dropdown list with one value “Go” in A2. I want to make possible for users to choose “Go” unless one scenario. In case they choose X1 or X2 from a dropdown list containing X1,X2,X3,X4 in cell B2, they are obligated to fill in a value in C2 before they can choose "Go" in A2.
it could be cool to show an error message telling you to fill in a value in C2 before you can choose "Go" in A2

Does that make sense

Hope for your help