From our friends in Columbia University Commencement/CUIT:
The ticket registration site is experiencing technical difficulties, and we hope to resolve this soon. Rest assured, there are plenty of tickets left for everyone. We hope to resolve this in the next 48 hours.
For students who have registered and did not receive their confirmation emails, you will receive it within 48 hours. If you do not receive it by Monday, please contact us at commencement@columbia.edu.