Why stack set cannot specify a sub-ou id to update even if there is a stack instance under the corresponding region

First, I created a stack set with service-managed permission model and set auto deployment. And I specified the root ou id to create stack instances, and then moved one user under one of the sub-ous (called ou1) to another sub-ou (called ou2) which was originally empty. Meanwhile, an update operation was automatically created. The question is, I wanna try to use this ou2 id to manage this stack instance related to the user who just was moved. But I still get an error : "Organizational unit [xxxx] has no instances in [region]". So, why i cannot give this ou2 id to manage this stack instance?