1.5 year after removal of sebaceous nevus from my head, there is no hair growth. What should I do for its growth?

Based on the size of the scar, you'll probably need to get a small hair transplant and/or SMP to properly hide it. I suggest seeing someone in person for a consultation and more specific recommendation.

You likely have a scar which is devoid of hair. Your best course of action is to see a hair transplant surgeon locally to discuss all of your surgical and nonsurgical options.  

Generally, a scar forms anytime and overtime an incision is made deep enough in the human scalp. The scar can be small or large depending on the procedure type and the patient's inherent skin characteristics. Removal of moles and other types of benign or malignant skin growths often lead to a clinically apparent (visible) scar - and always lead to formation of scar tissue under the scalp. Hair does not grow in a scar. The only means to increase hair density in such a situation is via hair transplantation surgery.