Almost perfect. Codzienny is an adjective and you need an adverb here. The whole phrase should read (including Polish diacritical marks and propper capitalization): Śnię o Tobie codziennie.
Nevertheless, I'd use: Śnię o Tobie co noc/każdej nocy. (I dream about you every night).
Note: if you mean dream in the sense of daydream the translation should read: Marzę o Tobie każdego dnia. = I'm dreaming about you everyday.
I think you meant "I'm dreaming about you every day."
"everyday" is an adjective in English like Thomas1 explained that "Codzienny" is an adjective in Polish.