How about: Wij kunnen bij de snackbar een portie friet bestellen (?)
can't we say "een portie frietjes" instead?
Yes, you can say that as well! ;)
I thought "een frietje" referred to a portion of fries, but it was marked incorrect. Am I wrong? :)
You're definitely not wrong: in this context (snack bar), "een frietje" should be accepted! So please go ahead and report it :)
In a restaurant though, it's more common to order "een (extra) portie friet(jes)".