I was trying to make an account for my younger sister, but I never got an registration email on the address, does anyone know about this?

any valid email address should work

i'd suggest checking her spam inbox or clicking the resend email button

Yeah, I couldn't find it in any folders

I'm assuming this .org address is a school assigned address. I'm not aware of any email hosts that provide .org's, though I'm only familiar with .com's. Work & School accounts can have emailed content restricted from being recived via settings by the work / school's administrator. Chances are this is probably a school account and the district's administrator has prevented outside emails from coming in. That, or the email address was typed in wrong, e.g. uppercase i's instead of lowercase L's used in the email.

According to https://scratch.mit.edu/faq, it says:

How do I confirm my account?
After you create a new account on Scratch, you’ll receive an email message with a link. Just click the link to confirm your account. Once you confirm your account, you’ll be able to share projects, write comments, and create studios. Confirming your account also lets you receive email updates from the Scratch Team. If you cannot find the email with the confirmation link, check your Spam folder. If you still can’t find it, and want to receive another copy, go to your Account Settings, click the Email tab, and follow the instructions there. Please note that it may take up to an hour for the email to arrive. If you still don't see the email after an hour, let us know.

If the “.org” email is a school email it might not work.