Entering a condition to perform SELECT in an existing report in SAP system

You have made one of the most common mistakes. In ABAP almost at all the places be careful with SPACE. You need to have a space in method calls so just have a space before and after the brackets as below −

SELECT SINGLE * FROM EKPO
WHERE EBELN = GT_MSEG-EBELN AND EBELP = GT_MSEG-EBELP
AND NOT ( F1 = 'value' AND F2 = '0' )