How could Kakashi make a Perfect Susanoo with only two MS when Sasuke needed a Rinnegan? When Sasuke first activated the Susanoo with 2 MS it didn't really resemble the Complete Susanoo. But when Obito gave Kakashi his eyes he activated Complete Susanoo. Why did it take so long for Sasuke to activate Complete Susanoo.
Once a user awakens the Mangekyō in both eyes, or the ability of both of their eyes, they are able to perform Susanoo.


Sasuke could make a "perfect" Susanoo once he got ability of both of his eyes. One probable reason why he didn't use it right away could be the fact that Susanoo does use up a lot of chakra and he might be saving his chakra.


  The Mangekyō Sharingan's abilities drain a considerable amount of chakra when performed.


Source


Mangekyo Sharingan