すみません、先約があります。

すみません、先約があります。

I'm sorry. I have another appointment.

すみません、先約(せんやく)があります。

Sumimasen, sen'yaku ga arimasu.

済みません (すみません)

excuse me; pardon me; I'm sorry; thank you

先約 (せんやく)

previous engagement; prior contract

が (が)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

有る (ある)

to be (usu. of inanimate objects); to exist; to live; to have; to be located; to be equipped with; to happen; to come about

Readings: セン、さき、ま.ず

Meanings: before, ahead, previous, future, precedence

Readings: ヤク、つづ.まる、つづ.める、つづま.やか

Meanings: promise, approximately, shrink