Why can't I move these artifacts into this planning folder?

To avoid confusion, you must observe a few basic rules when you assign artifacts to planning folders.

When an artifact in a planning folder has a child artifact, the child artifact can only be assigned to the same planning folder as the parent artifact, or a sub-folder of that folder. For example:

Before you assign an artifact to a planning folder, make sure the artifact and all its children are assigned to a single planning folder and its sub-folders.
Don't assign an artifact to a planning folder that is above the artifact's parent artifact in the planning folder hierarchy.
Before promoting a planning folder to a higher level in the hierarchy, make sure its member artifacts (and its members' parent artifacts) are all assigned to the same planning folder or a sub-folder of it.

An artifact can only be assigned to a planning folder in its own project.

As a best practice, consider instead creating a matching artifact in this project and associate the two artifacts.

Before you assign an artifact to a planning folder, be sure you have permission to edit every artifact affected by the move.