You know the selective service applies to 20 or something years-old people, right? I seldom see people who can’t apply for citizenship due to the selective service requirement. Besides, I am pretty sure it can’t stop you from becoming a citizen. Hope you can get a better answer from other people.
I’m a very recent immigrant, 23 yrs old who just got my greencard
upon my greencard interview, I was notified that not signing with selective service system is a felony, could be punished with 5 yrs of imprisonment and $250K fine, and denial of federal job and naturalization.
I don’t know how they would find out this, but just warning you. I suggest you to consult a lawyer, just in case. Good luck!