What can I do if I went to a clinic with a sore sebaceous cyst on my head so they sprayed it with a can instead of an injection and now my hair doesn’t want to grow in that area?

First, go to a doctor and get a diagnosis or prognosis is the hair expected to grow back, even it takes several weeks or a few months? If so, you have no viable claim, since the amount of money you could potentially receive for a temporary patch of hairlessness is far, far less than the cost of a malpractic lawsuit. Malpractice suits tend to be expensive, since you have to hire a medical expert witness to do a report and testify, and medical experts do not work cheap.
If there hair loss is permanent and some doctor or doctors confirm that the  loss was due to the spray, you may have a viable malpractice claim
1 It would have to have been, in light of current medical knowledge and treatment standards, known that this could happen so it was a risk the care providers knew about or should have known about but ignored and the risk of not treating the cyst must not have greatly outweighed the hair loss risk since a doctor can do something with negative effects on a patient, if the risk of not treating the condition is worse and 
2 it must be a sizeable areat that will not grow hair, since a dime- or quarter-sized patch would not even begin to support receiving enough compensation as to justify the cost of a lawsuit.