If you have been invited to an event and you cannot make it, here is a template you can use to write a letter to decline an invitation.
I regret to tell you that I won’t be able to make it to your event. Apparently, I already have prior commitments on that day. Kindly tell me how I can make it up to you. This is not a sign that I’m trying to avoid you though. If you have future events and I am free, I would love to go.