Hello, the following are original examples for your reference:

Dear Manager Williams:

Thank you for inviting me to your company's fifth anniversary celebration. Unfortunately, I have an urgent business trip to another city. I hope you can understand why I can't attend.

Thank you again for your invitation. I hope our long-term cooperation will be smooth and close.

You are sincere,

xxx

Answering business invitations focuses on conciseness and clarity, and the words are rigorous. Explain why you can't go in one sentence. Finally, I wish the activity a success or a happy cooperation. Just sign your standard name sincerely.

I hope it helps, thank you!