"A single plate"
Translation:Een enkel bord
For plural nouns, I would say they are interchangeable. However, for singular nouns the word "enig" has shifted very strongly towards its second meaning, "lovely" or "exquisite", and is rarely used in the meaning of "enkel" (a single ). There is one exception I can think of: "enig kind", meaning "only child" as in, one without siblings.
"Bord" is a het-word, hence with an indefinite article it does not get a -e suffix. As your answer was incorrect, you get a rather random example of a correct answer. That really is just an example; there doesn't have to be any relation with the answer you were trying to give except for being a translation for a meaning of the exercise. In your case, the example is about the literal translation of "metal plate." If you would make the same mistake again, you're likely to get a different example, possibly one that does have "bord" as example.