The latter is incorrect. You can say "Mada prezento wo kaimasen" "Mada purezento wo katte imasen" sounds like you have will to buy a present but you don't yet. "Mada prezento wo kaimasen" sounds like you think it's not time to buy a present.

Both sentences are grammatical meaning is correct.mada,purezento wo katte imasen.( まだ、プレゼントを買っていません。)That shows the current situation.(mada,purezento wo kaimasen deshita)まだ、プレゼントを買いませんでした。That means ``I dared not to buy the present (now)`` The nuance is little a bit different.