"He is a father."
Translation:Is est pater.
but handy when it's a beginner lesson, to learn pronoun, like it is here.
When a sentence is not accepted because you omitted the subject-pronoun, or because you included it. Please, just hit the report button, and the sentence will be added later as an alternative solution.
You seem to be addressing the course contributors. They don't typically hang around in the learners forums and comments here do not get sent to them.
If "Is pater est" was rejected, please flag it in-lesson and report "My answer should be accepted."
Keep in mind, though, that while Latin is indeed mostly SOV, copular constructions with 3rd person pronouns are more likely to be SVC (C=complement) because "Is pater est" can also be read as "This father is."