The Move Description command can only be used when labels are visible on the layout. Labels will not be visible when the Label Scale is set to a smaller value than the current drawing scale or when no items are enabled in the Label Enable section of the Display Options dialog.

Check the Display Options dialog (Options>Display) to determine if;

the Label Scale is set to a value smaller than or equal to the current drawing scale.
at least one attribute within the Label Enable section is selected. You will of course want to ensure the type of label you wish to move is selected.

Refer to Move Description for detail regarding Description movement or Display Dialog for detail regarding Display attributes.