Booker's Seven Basic Plots apply to many works of literature, but some texts just don't fit into any of these templates, and Sula is one of them. Maybe it's the unconventional plot, or maybe it's the fact that so many characters fulfill so many different roles.
That said, we can see aspects of several of the basic plots at work here. For example, in the Voyage and Return plot, the first stage is "anticipation and the fall into another world." Booker tells us that in this stage, the hero or heroine has a "consciousness [that] is in some way restricted." This sounds like Nel at the beginning of the novel, before she goes on the train trip. And the fall into another world? Nel's friendship with Sula does introduce her to a new world of sorts – Sula's world, with its busy, chaotic household and mother who is nothing like Nel's.
Or how about the Quest plot? In the "Arrival and Frustration" stage, the "hero [or heroine] arrives within sight of his [or her] goal" but is faced with new difficulties that prevent him or her from reaching it right away. If we turn to Nel again, we see that she does encounter these difficulties in her quest to forgive Sula. She goes to Sula's house when she hears that Sula is ill, and the two end up in an argument. She goes to Sula's funeral when virtually no one else will, but she still can't forgive the betrayal. It's not until Sula visits Eva in the senior home and then visits Sula's grave that she finally realizes how much she misses her dearest friend. But it takes a lot of agony for her to reach this point.