For various reasons, navigation is commonly simplified to a 2D grid, or as is the case in UE4, a set of 2D grids. These 2D grids are commonly referred to as “NavMesh” and are automatically generated based on the physics environment (within the nav mesh bounds volume).
This, however, means that 3D navigation is inherently impossible. You can use things like stairs or terrain to change elevation but you can not have your AI move in free space. To view the nav mesh you can press “P” within the viewport.
The MoveTo node attempts to find a path within the acceptance radius (which is in your case 5 units). 200 units above the nav mesh is therefore not within the acceptance radius and no valid path can be found.
For floating characters, consider having it move on the floor but not collide with anything dynamic, while displaying the mesh 200 (or however many) units above that.