The volume that is being shrunk is a secondary in a Metro Mirror or Global Mirror relationship, and the request to shrink would make the volume either larger or smaller than the associated primary volume. 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.
If you want to shrink the secondary volume, the secondary volume must be larger than the primary volume, and you can shrink it to the size of the primary volume only. When both volumes are the same size, you can shrink the primary volume first, then shrink the secondary volume to the same size.