"Some friends are irreplaceable."
Translation:Manche Freunde kann man nicht ersetzen.
I tried the same as you and it was also not accepted, but I persisted with the alternative 'Man kann manche Freunde nicht ersetzen' and that was fine, so you were thinking along the right track. I guess you have to directly negate the action of 'replacing' and not include the direct object.
Doesn't "(Manche) (Freunde) (kann) (man) (nicht) (ersetzen)" literally mean "(Some) (friends) (can) (one) (not) (to replace)" = "One cannot replace some friends?" This is not the same (to me) as "Some friends are irreplaceable."
Manche Freunde sind unersetzlich is a much better translation.
You are right in that "Manche Freunde kann man nicht ersetzen" literally means "One cannot replace some friends". However, using this active construction with "man" is much more common in German than in English, so that the sentence sounds much more normal in German and can well be accepted as a translation.
But, indeed, "Manche Freunde sind unersetzlich" is the literal translation of the given English sentence. And of course it is accepted as well.
I really appreciate you pointing out the differences in active construction between different languages (e.g., double negation in the Romance languages comes to mind), and I do think it is good to be exposed to different active constructions (in addition to the most literal and direct translation).
Thank you for the speedy reply. Here is a lingot for you.
"mousing over the words" is not intended to give away the solution. It just gives hints like a dictionary, namely translations of single words. Not all of them necessarily fit into the given sentence, and sometimees the words don't even appear in the translation, because different languages use different words.