why does the Mosaic keep telling 'you do not have valid appointment at this time'? I checked the prerequisite for the course, and i'm satisfied for all requirements.

Course selection on Mosaic is by specific appointment times. It sounds like that's the issue, rather than the course itself. If you are logging on at your personal specific appointment time and still getting an error, then you should contact your faculty or the registrar.

Edit: I just checked and course enrollment hasn't even started yet, so that's likely why you don't have a valid appointment right now.