The obvious question is have you got the master page selected that you wish to delete?
Are you able to drag the master spread to the trash icon?
If you only have two master pages, that is [none] and A-master, and you did not use the A-master for some reason, you only use the [none] then you have no option to delete any of those master pages.
However if you have too many master pages, try to double check if you really did not use a particular master page by going to flyout of Pages -> Master Pages -> Select Unused Masters
After that go back to Pages -> then Delete Master Pages.
Hope it helps.