Firstly we've moved this to the correct sub-space for the question. As you are using the netlist flow two things to check, firstly have you added a PARTS=20 property to the symbol to indicate this should be made up of 20 pins? Next, check the length allowed for the # property by opening the property definition editor from Tools - Property Definition editor. use the 'Advanced' option to see what length is set, by default it is 32 characters, change this value to something larger (max is 255) and you should be able to then enter the values you require.
Thanks Rob, It works after I changed the limit to 255.