Diagnosis
The quantity has no unit of measure assigned to it.

Procedure
In the field catalog, maintain either the field QFIELDNAME or the fieldQUANTITY for the field &1 of your internal table.
The field QFIELDNAME of the field catalog should be assigned the valueof a field in your internal table which contains the unit of measurefor the row.
Example:
data: begin of itab occurs 0,
quantity type de_quan,
my_unit type de_unit,
end of itab.
lt_fcat-fieldname = 'QUANTITY'.
lt_fcat-qfieldname = 'MY_UNIT'.
You can directly enter the unit of measure in the field QUANTITY of thefield catalog. This unit of measure is then valid for all rows.
Example:
lt_fcat-quantity = 'ST'.