Q: How do I decline an invite?
A: When you receive an invitation to something you don't want to go to or can't, the best thing to do is be upfront about it. What you don't want to do is tell the person you plan to be there (out of pity) and then either cancel or be a no-show.
Instead, thank the person for inviting you, and tell them that you are bummed about not being able to go. You can simply say you aren't able to attend or give a reason why you aren't able to attend. I would advise not to "make something up," though. Being caught in a lie is worse than declining an invitation.