Please  execute the following data-fix to make Tax Amount Zero on the AP Invoice in R12
UPDATE    ZX_LINES
   SET UNROUNDED_TAX_AMT = 0,
       TAX_AMT = 0,
       TAX_AMT_TAX_CURR = 0,
       TAX_AMT_FUNCL_CURR = 0,
       CAL_TAX_AMT = 0,
       CAL_TAX_AMT_TAX_CURR = 0,
       CAL_TAX_AMT_FUNCL_CURR = 0,
       CANCEL_FLAG = 'Y'
 WHERE APPLICATION_ID = 200
   AND ENTITY_CODE = 'AP_INVOICES'
   AND EVENT_CLASS_CODE = 'STANDARD INVOICES'
   AND TRX_ID = 90484;
   
COMMIT;
UPDATE    ZX_LINES_SUMMARY
   SET TAX_AMT = 0,
       TAX_AMT_TAX_CURR = 0,
       TAX_AMT_FUNCL_CURR = 0,
       TOTAL_REC_TAX_AMT = 0,
       TOTAL_REC_TAX_AMT_FUNCL_CURR = 0,
       CANCEL_FLAG = 'Y'
 WHERE APPLICATION_ID = 200
   AND ENTITY_CODE = 'AP_INVOICES'
   AND EVENT_CLASS_CODE = 'STANDARD INVOICES'
   AND TRX_ID = 90484;
   
UPDATE    AP_INVOICE_LINES_ALL
   SET AMOUNT = 0,
       BASE_AMOUNT = 0
 WHERE INVOICE_ID = 90484
   AND LINE_TYPE_LOOKUP_CODE = 'TAX';
COMMIT;
UPDATE AP_INVOICES_ALL
   SET TOTAL_TAX_AMOUNT = 0
 WHERE INVOICE_ID = 90484;
COMMIT;
