Note the word is "employee" with two 'ees" υπάλληλος this could be masculine ο υπάλληλος ot η υπάλληλος feminine.
Here's a set of good dictionaries. https://www.lexilogos.com/english/greek_dictionary.htm
It's more natural to omit "ένας". In fact, that's what you'd get to hear (almost exclusively). The thing is that the article has been included in some of the possible translations to better match the English article "a" so that it's easier for learners to match the two words. However, this leads to a more unnatural sentence. Not in all exercises have the two versions (with and without the article) been included, so there are a few inconsistencies throughout the course. We've started omitting it in the second tree. In this exercise, however, both should be accepted.