how to wan science victory in civ 5 original

Hi I am palying civ 5 ( not gods and king and not BNW) I think it called vanilla,
anyway I won a diplomatic victory and now I want to win other victory types, I have built all spaceship parts. When I finished two of them I remember there was a button which I printed and then a notification appeared that I built it but the 4 other the button is not there ? What to do? How to win?

Once you win a diplomatic victory (or a science victory or culture victory), the only victory you can get in "one more turn" time is domination. So, to get a science victory after winning a diplo victory, you have to start a new game -- you cannot do it in the same game.