For this case what we should do is to choose each of the expressions used in Spanish to reject an invitation.
We have then:
1) Me encantaría pero...
It is a phrase that usually comes with a reason that says why you will not accept the invitation
2) En otra ocasión tal vez...
In this sentence you are already saying that you can not attend the invitation.
3) Qué lástima pero...
It is a phrase that shows repentance, followed by an explanation of why you will not accept the invitation.
4) Cuanto lo lamento pero...
It is an expression that demonstrates the same meaning of expression number 3.