Use this phrase when you're giving someone advice:

You can't just give up!

You can't just show up to class the day before the test and expect to get caught up with everything.

You can't just pack up your bags and move to a whole new city, where you don't even know anyone.

Or you can scold or criticize them:

You can't just call in sick whenever you feel like it.

You can't just walk in without an appointment.