If you use a VUMAT subroutine for an element, it contains outputs that you define in subroutine. These outputs can be viewed in abaqus/viewer as SDV1, SDV2, ... and some special outputs such as S11, S22, ... . You should define SDVs (solution dependent variables) in your subroutine. Therefore you can not use other outputs for these elements. By the way, damagemc is an output that is caculated for shell elements, not solid element such as C3D8R. If you want not to see this error, remove damagemc from output request of these elements.