Er is the verb "to be," ex: Jeg er en mann (I am a man), and is not in the same category as en/ei/et.
In general, en is for masculine nouns, ei is for feminine, and et is for neuter. These are all indefinite articles (a/an in english).
The thing is, if you have a feminine noun, you can choose whether to use the masculine or the feminine article (en and ei). Considering jente is a feminine Norwegian noun, it’s therefore correct to say both ei jente and en jente. Remember that this only applies to feminine nouns. If you have a masculine Norwegian noun, you cannot use the feminine article in front of it. It’s therefore wrong to say for instance ei gutt.
In this specific case, eple is neuter, so we say, "et eple."