I am wondering if I do a certificate course which is only 5 months but an employer has guarantee me a job and work practicum. Will I get work permit upon completion of my certificate?
That's irrelevant. Again, your studies are too short to qualify for a post graduate work permit.
If that employer wants you to work for them, they will first need to obtain an approved LMIA. Once the LMIA has been approved, you can then apply for a work permit tied to that specific employer.