Sorting a paging grid column that is a nested CDT.

Try using the index and then dot notation if that child field is an array or else parent.child should work. I can address your issue more precisely if you can provide the exact CDT.

it looks something like
Id-Int
Sponsid-Int
anumber-Int
Contact-Type(Contact)-Multiple

The Sorting should happen on the Last name in the contact CDT

So do you want to display sorted multiple last names in a single row?

when i click on the sorting column it throws an error [contact.LastName] because it is a complex,multiple or child of a multiple data type.

when i click on the sorting column it throws an error [contact.LastName] because it is a complex,multiple or child of a multiple data type.

As There are multiple values in each row, Appian doesn't know which one to consider for sorting.
As far as I understood from what you said. Your data should look like this:
ID....contact
1....a,b,c,d,e
2....b,f,,h,j

so now, if you want to sort by lastnames(a,b,c,d,e..) Appian is not sure which one to consider. When the 'a' from id 1 is considered with 'b' from id 2 the sorting will not be same as when 'd' from id1 is considered with 'b' from id2.
Sorting can be done on columns which have only one element per cell.
Hope this helps.
Yeswanth.