I'm sorry I can't come.

对不起，我来不了了。

youdao

I'm sorry I can't come on Saturday, I already have something on.

对不起，星期六我不能来了，我已有约会在先了。

youdao

I'm sorry I can't come to the party because I have to attend a meeting.

抱歉我不能参加这个聚会，因为我的参加一个会议。

youdao