Persons incapable of forming melanin are called:

Persons incapable of forming melanin are called albino. Albinism is a genetic condition characterized by a lack of melanin pigment in the skin, hair, and eyes, leading to very light skin and hair, and vision problems. The condition results from mutations in genes involved in the production of melanin.

References:

Genetics and Pathophysiology of Albinism by David C. Bennett and Dorothy C. Cooper

Clinical Dermatology by Thomas P. Habif