The PowerSchool Community is a great resource if you have any difficulties completing fields within the enrollment form. I have attached an article here that will guide you through the process of selecting a school.
If you still face the same issue, I recommend reaching out to your district directly. The PowerSchool Admin at the district will be able to assist you with the process to select your base school while enrolling your child in school.
You can check out the FAQs related to the enrollment process here.