If you don't have the skills, you politely refuse the job. Or click on one of the reasons in the drop-down menu when you click on "decline", if you haven't accepted the interview.
Mahmoud, in addition to what Nichola shared I wanted you to know that if you're like many of us, you will receive Invitations whereby you don't qualify for the project as well as those you're not interested in. Of course Nichola's great suggestions can work in either scenario. Best of luck to you!