I'm currently enrolling in my courses for Engineering I and I've already enrolled in all of my mandatory courses for Eng I without issue. However, I've tried to select a couple electives and although Mosaic says that the course is open, it says that "Available seats are reserved and you do not meet the reserve capacity criteria". Does this mean that I can't take that course?

Eh, probably means most of the seats are meant for people in some other program, and whatever seats were available for Engineers are already taken...

First make sure I'm right, then contact the adviser and ask if you can be put on the waiting list. Seeing how 1st year Eng has so many people, I'd try taking something else or leaving it until summer.

Either way, it's not the thing I would worry about if I was 1st year Eng student.