In general the above should work (assuming the tables are OK). Since We do not have a screenshot or description of the model, we make a guess.
The most likely cause is that you are using this command on 3D elements (e.g., HEX or TET) that have only UX, UY, and UZ dof and thus accept only forces, FX,FY, and FZ (so no moments - hence the mx label is not applicable for these elements - it is to be used with shells and beams only that have rotational dof).
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