No, and your eligibility is not negatively impacted if you cannot accept the invitation to sign-up.
Sometimes, Scholars must miss a year in the middle of their DECATS eligibility window because of other summer activities, or a new nominee is not able to start in the first year of eligibility due to other commitments.
Once you are nominated, you are always eligible until you naturally age-out of the program. You do not need to be re-nominated by your school. Scholars who have attended the program before are still considered returning Scholars and will be invited back in that group of participants until they naturally age-out of the program.