Hello
Flat structures contain only fields of the elementary types C, N, D, T, F, I, P, and X, or structures containing these types.
Flat structures contain only elementary data types with a fixed length (no internal tables, reference types, or strings).
In a flat structure, every field either refers to a data element or is directly assigned a data type, length and possibly decimal places.
Try to change ur field type.
Reward points if helpful....
Regards
shailendra