I was unable to attend because of a previous engagement.

我因有约在先，无法出席。

《牛津词典》

youdao

youdao

Somalia's weak central government must deal with a deadly rebellion inside the country and is unable to police its waters.

VOA: special.2009.04.25

Why are we unable to talk to a crowd as we talk to a single friend?

为什么我们不能跟一群人讲话,却可以单独同朋友高谈阔论？

耶鲁公开课 - 心理学导论课程节选

Or, for that matter, where a loved one of mine is unable to make the decision,

或者，在某种情形下，我的某个亲人无法做出这个决定，

对于安乐死的看法 - SpeakingMax英语口语达人

Meanwhile, in Wisconsin, Democratic senators have fled the state, leaving the State Senate unable to reach a quorum and consequently unable to vote on a budget proposal that would drastically reduce the power of unions in the state.

FORBES: Fleeing Wisconsin Senators Find An Ally In Howard Dean

Speaking from my own experience, I recently shut down one deal when a potential partner was unable to consider my time as important as his, and another when a potential ally was unable to return calls in a timely manner.

FORBES: The Peace Of Mind Of Regret-Free Living

Manning entered training camp on the Physically Unable to Perform (PUP ) list, a category reserved for players unable to pass a physical entering camp.

FORBES: The Puzzle Of How The Colts Are Handling Peyton Manning