The "Evening Star" is not a star but the planet Venus.
The "Evening Star" is not a star but also the planet Venus.
The "Evening Star" is not a star but the planet Venus.
The "Evening Star" is not a star but also the planet Venus.
The paired structure "not ... but" places two items in parallel. The two pieces, each a single word, must come together: one without the other is incorrect, and variations (especially confusions with one piece of "not only ... but also") are not correct either.