To get an alliance you need to have 9 points of "trust" with a race, usually by promising friendship, appointing diplomats and forming trade routes with them. After some turns if the race leader will call you saying that the relationship between you two has improved you'll get one point. Once you have 9 you can propose the alliance through the trade interface.

Thanks for the help.

Nice game. Fine strategic depth, and superb strategic "feel". Quite refreshing after the overpriced, misdesigned, and undersupported mess that is Civ 6.