The volume that is being shrunk is a primary volume that is already smaller than the associated secondary. Volumes in relationships can be shrunk by first shrinking the primary by the required amount, and then shrinking the secondary volume to the same size.
Either shrink the secondary volume to be the same size as the primary volume, or expand the primary volume to be same size as the secondary volume. When both volumes are the same size, you can retry shrinking the primary volume.