Either Der Wein ist gut. (with marked emphasis / intonation on der) or Der Wein dort ist gut. (with dort as adverb "pointing" to the wine in question) or Jener Wein ist gut. (but that may sound archaic or pretentious as jenes (and other forms) is rarely used these days except in literary writing).
You can only report it immediately after you got marked wrong on a sentence.
So if you had already gone further from the exercise, you'll have to wait until you get that question again (e.g. while strengthening this exercise), put in the "wrong" answer again, and then when it tells you it's wrong, you'll see (on the web version) a button "Report a problem" at the bottom left - click on it and one of the options in the pop-up menu will be something like "My sentence should be accepted".
On the iOS app, there's a little "flag" icon on the "sorry, that's wrong" notice that can also be used to send a report.
I don't know about the Android app, sorry.
gut is the basic form of the word -- the form you would look up in a dictionary, for example.
When it's used as a predicate adjective (roughly: after a form of the word "to be"), then it's used in that basic form, without any ending: der Wein ist gut.
When it's used as an attributive adjective, though, before a noun, it needs an ending of some kind.
The specific ending will depend on the gender, number, and case of the noun as well as on what kind of determiner, if any, is before the adjective. For example, guter Wein "good wine" with strong ending -er but der gute Wein with weak ending -e because of the preceding definite article der.
"goof" is a valid English word, just not the right one in this context.
Duo has no way of knowing whether you used this wrong word deliberately or whether your fingers slipped, so it marks it as a mistake.
Typos can only be detected if the result is not a valid word.
But turning good into goof or food or hood will result in a mistake because those are all valid words.
Why is wine in capital letter?
It isn't. "wine" is lowercase.
Wein is uppercase because it's a German noun, and all nouns are capitalised in German.
This feature of German spelling is so basic that it's mentioned in the tips and notes for the very first lesson unit, https://www.duolingo.com/skill/de/Basics-1/tips-and-notes .
Haven't you been reading the tips and notes? Then you've been missing out on a lot of grammar explanations.
If you're using a mobile app, stop. Use the website https://www.duolingo.com/ instead, at least for learning new material. (An app can be helpful for reviewing material you've already learned.)
On the website, you can choose to view the tips and notes before you start each new unit, and I highly encourage you to do so.