"His name is Luke."
Translation:Han heter Luke.
First of all, you should inflect the noun, so it would be 'navnet', as the possessive comes after. Secondly, 'sin' are wrong for two reasons: 1. 'navn' is neuter, so it should be 'sitt', 2. 'sin' refers to a subject, but in this case 'Luke' is not a subject, so it should be 'hans'. Using 'sin/sitt' is wrong.
"Navnet hans er Luke" is accepted.