Forming a proper question in English is complicated. In such a question that includes a question word, you only need an auxiliary verb if the thing you ask about is not the subject.
Asking about the subject:
- How many people work here?
- Who slept in my bed?
- What bothers you?
Asking about the object; the subject is marked in italics:
- How many people do you employ?
- Who did John sleep with?
- What do you do?
I agree. If you don't get a chance to see what you entered you may miss your mistake. For instance, you may have put the words out of order and were marked wrong but you don't get to see your error because your answer disappears.
It is not exactly wrong, but it's not the most common way to form this question. Usually, if you ask about the subject, you just use the full verb:
- How many men study here? - "Men" is the subject.
- What do the men study here? - "Men" is the subject; "what" is the direct object.
- Where do the men study? - "Men" is the subject; "where" is an object.
When Duo spots a mistyped word in your sentence, it runs a search through all other words in its database for a word that matches yours exactly. If it finds one, it'll grade your sentence as wrong because you might have deliberately translated it wrong.
Normally when you have a gender mismatch (like writing "*cuántas hombres" here instead of "cuántos hombres"), it'll mark you off, since both cuántas and cuántos are in the database as different words. If it grades it as a typo, that can have two reasons. Either there is no counterpart of the other gender in the database, or the program can't connect to the server to run the search properly.