"My boyfriend is very handsome."
Translation:Mi novio es muy guapo.
The "personal a" is used for the direct object (and a is also used elsewhere).
Amo a mi novio. I love my boyfriend. Vi a Juan I saw Juan.
You might have seen A mi novio in a sentence like this: A mi novio le gusta el café My boyfriend likes coffee
You're getting a little confused by the use of the Spanish 'a', which is understandable as it has many uses.
Going back to your original question: why is 'A mi novio...' not correct here?
It's not correct because 'novio' (boyfriend) is not the direct object here. The personal 'a' is only used when the direct object is a person.
No conozco a tu padre, I don't know your father.
Quiero comprender a mi profesora, I want to understand my teacher.