It simply sounds wrong. When you're describing a total lack of knowledge, you pretty much always use the idiom '[...] do(es) not know [...] at all'.
My answer was 'i don't know you completely'. Isn't this grammatically correct. Its meaning might be different though. I guess my answer means that i know you but not very much. Is this understanding correct, english wise.
Yes, it's grammatically correct and you are right that the meaning is different. Similarly, in Dutch that would be: "Ik ken je niet helemaal".
Basically, the position of the adverbs "helemaal" and "completely" depends on whether they modify the negation, or the verb itself.
The speaker pronounces "ken" in the same way a dutch speaker would pronounced "kun".
How about 'i do not completely know you'. ?It sounds right but it was marked incorrect
The sentence has a different meaning, just like the sentence AsmaaMagdi wrote above. It would mean you have some knowledge of the person (though not complete), whereas "I don't know you at all" means you have no knowledge whatsoever of the person.
Because that means something entirely different? The Dutch equivalent of that sentence would be: "Ik ken je niet eens."