why is "how's the river?" wrong?
I agree with you, how's the river and what's the river like are pretty much the same.
"Milyen" is a question word that means something like "of what sort?" The duolingo translation seems to be "What is ... like?", which is pretty close.
Come to Kisoroszi and have a good swim and I will ask you "How is the river" not "what is the river like"... (Kisoroszi is a small village upstream Budapest, and has nice places where you can swim in the Danube quite safely.)
If you were swimming and asked "Milyen a viz?" wouldn't it be asking - "how's the water?" not, "what is the water like?"
In this context, I think both would be appropriate. If I understand right, both would be a good translation of "milyen a viz" in that context.
Would something like "what kind of river is it?" or "what type of river is it?" work here? "how's the river?" is something I can't imagine asking someone.
"Milyen a vmi" (vmi = valami = something) means "What is [the thing] like?"
"Milyen vmi (ez)" is "What kind of [thing] is it/this?"
The a is an important factor here, determining whether you ask about a certain thing, or a general group of things.
It's missing the word "like" on the mobile app so it's impossible to leave the correct answer
Why ot can not be "what kind of river is ?
That's not a complete sentence. There has to be some noun after "is".