Cannot sort by field because it is from a one-to-many record relationship.
Discussion posts and replies are publicly visible
Sorting is not supported for one to many relationship columns in read only grid. If you are trying to display multiple values of child table for one row of parent table then it is not the best fit for read only grid.Instead, you can try rendering the grid on child table and also display columns of parent table using many to one relationship. this way sorting will work.