Volume elements, such as solid186, only have translational degrees of freedom (DOF), no rotations. However, the model still needs to be constrained from rotating. This is done by having some constraint in XYZ directions for at least 3 non-colinear nodes. The UZ=0 on the plane of nodes at Z=0 will prevent RX, RY rotation. However, you then restrict the node selection to all nodes along the Z axis and set, UX=0, UY=0. This still allows RZ rotation. If you set UX=0 or UY=0 on just one node not along the Z axis, you will then have a fully constrained model.