Can you get financial aid with a certificate of completion?

Yes, you can.

More in depth, although, it depends on your financial need.

If you live in America you need to fill out a FASFA (fasfa.ed.gov) and list the school you wish to attend on there.

If you are eligible for financial aid, your school will offer it to you.

Generally, the easiest way to find out if you're eligible is to talk to the community college's financial aid office.