Why is use of pronouns marked wrong?

Maybe this had been discussed before, but I am new here and could not find the topic while browsing through the forum.

I am having problems with Memrise not accepting personal pronouns when translating ENG -> ESP. E.g., I translate sentences like “He is good-looking” with a personal pronoun: Él es guapo.

However, Memrise marks it wrong, and gives Es guapo as (only) correct answer.

I know the use of pronouns is not mandatory, but omitting them is not mandatory either, so it should not be marked wrong. Or am I missing something…?