"The doctor makes the professor healthy."
Translation:Medicus professorem sanum facit.
Maybe the question is about "make healthy" /"sanus facere"?
It seems idiomatic in English, and idiomatic in Latin (not every languages use this kind of expression, and I don't think it's used in descend languages (?), but "sanus facere" is real and not a calque from English.
"aliquem sanum facio": https://www.online-latin-dictionary.com/latin-english-dictionary.php?parola=sanus
Latin is weak at forming causative forms and that what it often uses instead of them. A causative, e.g. of "to fall" is "to fell" (to cause, for instance a tree to fall). That's the only or one of a few grammatical matters related to verbs where Germanic languages are more advanced.
Languages evolved different. I studied Sumerian out of the John Hayes' text. Sumerian has some interesting modal constructions that provide extra information about the context. Middle Egyptian had prefixes that could be added to verbs to make them causative. It doesn't meen one is more advanced, just the rules and syntax developed in other manners.
If a makes b c, respectively if the subject makes the accusative object a complement, both b and c are in the accusative. -um is the accusative suffix of words that usually end in -us, like sanus.
But I guess you meant the verb. Medicus professorem sanat. That should be correct, too, but then the doctor would heal him, and not make him healthy; however, someone healed is someone healthy and if Duolingo complains about that, I would find it a bit hidebound. Did it?