There is both a neuter and a masculine version of the noun "øl".
The masculine one is used when referring to a glass, bottle or can of beer, and is what you'd use to order a pint at a bar. The neuter is used to refer to the beer itself, be it in a general sense or when referring to a specific type of beer.
Your second statement is incorrect; "en" can replace "ei" if you opt to decline feminine nouns as if they were masculine, but neuter nouns do not afford you the same option.
Thanks so much for the explanation. I stumbled over that as well and thought there was a mistake in the exercise.