Velocity render element is not supported on meshes with changing topology - velocity will not be rendered for such meshes.
This message occurs when the Cgeometry_Velocity render element is present and enabled, and the scene contains meshes with animated changing topology.
The Cgeometry_Velocity render element will be rendered only for objects without a changing topology.
Presence of mesh(es) with animated changing topology in scene with an active Cgeometry_Velocity render element.
Unfortunately there is no known solution to reliably compute topological motion blur on meshes with changing topology, unless motion vector data were computed in advance and stored within mesh vertices.