“I'm sorry, but I will be unable to attend the meeting tonight.” If necessary, you can add a vague reason: “because of a family emergency,” “because I am stuck in another city,” “because I have a prior engagement,” etc., but only if the meeting warrants that.

https://www.quora.com/What-is-the-correct-way-to-say-I-will-not-be-able-to-attend-a-meeting-tonight