From LimeSurvey Manual

If the super-admninistrator chooses to have the "No answer" option displayed (from the global settings), the Show "No answer" of the local survey field becomes non-selectable because the global settings trump the survey settings (idem for the Off case). To offer freedom to your survey administrators, the super-administrator has to choose Selectable for the Show "No answer" option from the global settings of your LimeSurvey installation.