how to enroll into classes/units?

so i think i enrolled in 2 required units for term 1 in my standard foundation program. but im confused about how it wont let me schedule nor enroll into classes in other terms. It wont let me enroll in classes in other terms and im also confused about how many units im supposed to enroll in a term/semester? or do i have to do the unit within the whole year? I need guidance pls help 🙏🏻