Why can't we say "seni" instead of "sana" ??
"sana" is used for recipients/indirect objects if you will. "I give you something/I told you something." "seni" is used for direct objects "I gave you to him/I kissed you"
Does 'Et sana vermiyorum' sound weird?
Yes, indefinite direct objects (which don't have any case ending) must come next to the verb.
So 'Eti sana vermiyorum,' with a definite direct object, would work just fine?
Yes but it would mean I am not giving you THE meat.
What bad has done "ben" at the first of the sentence because of the word "i " that is not correct .is it grammatically wrong or ,what ?