I am using Mystic elf race(dawnstar elf) mod so i suspect this is the case. However when i tried a new character as a nord, i couldnt feed despite being a lv 1 vampire. The reason why i need to feed is because by feeding, it activates a vampire mod that i just installed.

I tried the console commands to remove vampirism and getting it again, but its no use. The 'feed' dialog simply doesnt show up when i go sneak mode and 'E' on the sleeping person.

Is it because:
-I am using mystic elf race? But the person who created this mod says that he only modified the looks of the race. The dawnstar elf looks like a nord with elf ears and shorter than nords only. And given the fact i could not feed as a nord on my other char shows it isnt this fault, but who knows
-Can a level 1 vampire feed?

If you're using a custom race, the game might not notice and not trigger turning you into the vampire race which would then prevent feeding.

yea i realised that. Issue solved.