das ist nicht meine Tochter
is that correct ? ain't i supposed to use "keine' since tochter is a word ?
Negating a noun:
"Kein" is used when the noun has an indefinite article or nor article at all.
"Er ist kein Arzt." "Dies ist kein schönes Haus." "Das ist kein Kaffee."
"Nicht" is put in front of the negated noun if it is preceded by a definite article or a possessive pronoun.
"Er ist nicht der Arzt." "Dies ist nicht mein schönes Haus." "Dies ist nicht meine hübsche Frau."
In your example, "meine" is the possesive pronoun. Therefore, "nicht" is correct in this case.
First of all, you posted this to the Italian forum. You should maybe post it in the German forum, you'd get more answers. Here's how to change it: https://www.duolingo.com/comment/16609773/How-to-move-a-post
Second, I can't really explain it (native speaker, don't know the rules), but "Das ist nicht meine Tochter" is definitely a correct sentence meaning "This is not my daughter". The sentence "Das ist keine Tochter" means "This is not a daughter", implying that the person you're talking about is not anybody's daughter.
"Keine Tochter" means "not a daughter," so to say "Das ist keine Tochter" ("That is not a daughter") would probably not be accurate, since every female is (hopefully) someone's daughter. You could say "Das ist keine Tochter von mir" ("That is no daughter from me", or more idiomatically, "That's no daughter of mine"), but that sounds more like you're disowning your daughter for some reason. So yeah, "nicht meine Tochter" is the most natural and neutral way of saying what you want to say.