I haven't played in a while, but i'm pretty sure that:
This part - "I do not have enough renown" is referring to the fact that you are not a vassal for that specific faction or that you don't own the city.
And this part: "birthright to own this land" is referring to the fact that you did not choose that particular faction in your character creation.

So you'll only be able to buy land in Riverrun (for example) if you're from the riverlands in character creation, or if you are a vassal for Edmure or if you the owner of the city.

Let's say you're from the north, that will mean that you can only buy land in barrowton, white harbour and winterfell in the early game, but if you go down and conquer fairmarket you will be able to buy land there then.

I think this is it, cause I remember having the same issue back in January when I last played this mod. I can't tell you with 100% accuracy but go on the discord server for the mod and ask it there.