Recently updated to 2017, never had this issue before but when I right click to lock a dimension it brings up a menu. It use to let me lock the dimension without a menu popping up. It is as if it doesn't know that I am placing a dimension. See pictures for a better understanding.
http://help.solidworks.com/2017/English/SolidWorks/sldworks/t_Locking_Dimensions.htm?id=401e64cbfaba487f9b490640651f97c5#Pg0&ProductType=&ProductName=