"Go ahead and take the cookies that you want."
Translation:Prendi pure i biscotti che vuoi.
I had the same issue. Turns out there is a different set of conjugations when giving commands. Check out this article:
I think the only mistake in your sentence is adding "ALL". I am not so sure of the use of "also" here, "pure" here means "well then".
"Well then, take the cookies that you want"
Maybe they are at the supermarket, and the kids want that special kind of cookies.
Your sentence would translate into
"Prendi anche tutti i biscotti che vuoi" (probably you already took all the candies that you wanted as well)
I don't really understand why it wouldn't be accepted since from the early begining, lessons use pronouns.
But! indeed, in italian, pronouns are often not used in common statements (Try not using them in translations, you will see it's accepted). Rather they are used to emphasis the statement :
Vado a scuola = I go to school
Io vado a scuola = I DO go to school
Anyway, as we don't have any context with this statement , 'tu' should not be used, but you couldn't know :)