If you're unqualified for something, you're not cut out to do it, often because you don't have the skills or knowledge you need. A dog trainer is most likely unqualified to perform brain surgery.
The adjective unqualified is good for describing someone who isn't fit for a task or job. You wouldn't want an unqualified dentist to fill your cavity, and an unqualified babysitter might not even know how to change a diaper or put a bandage on a scraped elbow. Another way people can be unqualified is when they lack some necessary document — you're unqualified to teach in a public school, for example, without a state teaching certificate.