For a non native English speaker, it's annoying when the program gives you the red light when you forget the word "a".... Well at the same time this is an English course.
In English, all nouns use an article if there is no noun determiner used (this, that, some, any, no, etc.), or if it is a plural indefinite noun. (There is no plural for "a.")
Still, as Polskandor noticed, it's also an English course. For the Polish people learning English, the so-called reverse tree. Whether we like it or not, the sentence isn't grammatically correct without an article.