"I don't follow back because I don't know you."
"ㅎㅎ 죄송하겠지만, 나 맞팔 안해요. 너를 모르니까." => 죄송하겠지만, 나, 너, 모르니까 are not right.
=> ㅎㅎ 죄송하지만, 저 맞팔 안 해요. 님을 모르니까요 [or 님이 누군지 모르니까요].
* 너 is very rude to say to an adult except close friends. 나 is also not nice. Must use 님 (honorific "you") and 저 ("I").
* 겠지만 softens the expression introducing an uncertainty. To say sorry, we shouldn't say we "might" be sorry with 겠지만 but just sorry unequivocally with the plain 죄송하지만.