"Nu är goda råd dyra" is a Swedish idiomatic expression, so it makes sense to us to use the plural form. And you can't say just "gott råd", it has to be "ett gott råd". So, what would "ett gott råd" be in English, "a piece of good advice"?
In another example, "Jag gav honom ett gott råd" (IIRC), "good advice" is accepted as correct for "ett gott råd". Should it not be in this case as well? I.e. should "Ett gott råd är dyrt" not be accepted?
It's not an accepted answer, but I seem to recognize this from other courses. Duo doesn't always show all errors. When there are several errors in the input, it will often just point out one of them, at least I've had that happen to me many times.