Do I need a job offer before I can enroll?

Not to enroll, no. You'll need a real job when you apply for CPT itself - the school reviews your offer letter and duties, and most use a CPT Agreement your employer signs. Most students enroll first and land a job in parallel. Also, for some universities, you must obtain a job within 90 days of enrollment. Otherwise, they will dismiss you. For example, New England College and National Louis University are two of them. So do you have a job offer in hand?