Hello , I'm having some problems with the new DLC Hearthfire , and I'm wondering if you people could help me.
I got this DLC intending to get Lakeview Manor House , but when I tried to buy it I realized it was impossible.

Every time I talk to the Jarl Seidgeiir I only get two options

1- Does your inexperience makes your job difficult?
2-Can't remember , but it also leads to nothing.

I've tried to help everyone in the city but the result is always the same. Some people said it's because in one of Nazir's mission I killed the Housecarl Helvard.
Is there anyway to fix that?

You're meant to address the steward, not the Jarl I believe. As I done so in Morthal.

I've talked to Nenya but don't appears the option to buy the house

have you done any quests at all yet for the jarl? his first quest is usually to make you fetch him some black briar mead. also where do you stand in the civil war? you may try checking the wiki if you havent done so yet already, also you may have better luck in the hearthfire subforum

Yeah, I think you have to do a rare gifts quest for that particular Jarl but make sure there isn't a similar quest that you are doing for anyone else in your journal as you will have to complete that quest before the Jarl of Falkreath will assign you his quest. Once that's done you should have to option to purchase land from the steward.