I have done the required quest and became thane but still can't buy the land for the manor. Can anyone please help

If you have the new Jarl "Sorli the Builder" she has a husband who will sell you the land, You have to ask him to buy a house.

If you don't see him in the house he will show up if you wait. His name is "Pactur"

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Pactur sometimes remains in Stonehills (his and Sorli's original home), however this appears to have been fixed with the Hearthfire DLC, so he should move into the Jarl's house (Highmoon Hall) in Morthal after a few days of taking Hjaalmarch for the Stormcloaks.