Hi @Jackie Dinh,
Per my research, you could use following code to create the folder in msgraph-sdk
var folder = new DriveItem
{
Name = "<sub folder name>",
Folder = new Folder()
};
var result = await graphClient
.Sites[siteId]
.Lists[listId]
.Drive
.Root
.ItemWithPath("<folder path>")
.Children
.Request()
.AddAsync(folder);
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