Yes, as is the rule, direct object is marked by accusative in positive statements and by genitive in negations:
- „Mam (kogo? co? accusative)mleko” but
- „Nie mam (kogo? czego? genitive)mleka”
Well, that's the most logical, basic answer. If it was rejected, it clearly must have been a bug.
do not = don't is accepted automatically by the algorithm.