"She has water."
Translation:Sie hat Wasser.
Because it is no "special" water. The English sentence doesn't have a "the" either.
"Ihr habt Wasser" if you were talking to a group of people "you all", but it is asking for "She has water" in which case you are talking about "she" singular.
einen is masculine accusative singular, but Wasser is not masculine but neuter.
Also, Wasser is normally uncountable, so we don't use it with the indefinite article -- just as we say "She has water" in English and not "She has a water", so in German we would say Sie hat Wasser and not Sie hat ein Wasser.
For the same reason that you can't just say "Why doesn't you just say doesn't you?" or "He am my friend but you is my friend too".
You have to pick the right verb form that matches the subject.
Here, the subject is sie and the appropriate verb form for that is hat.
hast would be used for the subject du.
Grammar is explained in the tips and notes for most units.
If you are using the website, you can find the tips and notes behind the 'lightbulb' icon next to the Start button, after selecting a unit:
If you are using a mobile app, you probably don't have access to the tips and notes. I would recommend that you switch to using the website instead (though the mobile app may be useful for revising/repeating material).
I have heard that the website might not display tips and notes when accessed on a small screen (e.g. on a smartphone); in that case, you may need to use a tablet or laptop or some other device with a larger screen.