When finding an excuse for not doing something, it's essential to be honest, polite, and respectful. While you want to say something nice, remember that honesty is important, and you should avoid lying or making up excuses. Here's an example of a polite way to decline a request:
"I'm really flattered that you asked me to [do the task], and I genuinely wish I could help. Unfortunately, I have prior commitments that I can't reschedule. I'm sure you'll find someone else who can do an amazing job!"