"Sorry that this letter is typed, but my handwriting is awful and I was afraid you wouldn't be able to read it due to the messiness!"
Here's a translation done in a casual style.
이 편지 타이핑을 해서 미안한데 그건 내 손글씨가 너무 서툴러서 (쓴 걸) 네가 못 알아볼까 봐 그런 거야! [or 그런 거니까 이해해 줘!]
* 손글씨: handwriting.
* 서툴러서: because it's poor.
* 쓴 걸 (쓴 것을): what I wrote.
* 알아보다: recognize; read.
* 이해해 줘: please understand.