在USING子句的查询中，您只包含SELECT pt.transaction_date，而不包含transaction_id，因此标识符是未知的。如果你想使用该列，那么你必须将其包含在SELECT子句中。

MERGE INTO PAYMENT_PLAN pp
USING (
  SELECT pt.transaction_date,
         pp.transaction_id
  FROM   payment_plan pp
         JOIN transactions pt
         ON (pp.transaction_id = pt.transaction_id)
         JOIN policy pol ON (PP.POLICY_ID = POL.POLICY_ID)
  WHERE  pp.payment_plan_status = 'CLOSED'
  AND    POL.PAYMENT_METHOD IN ('Cash','DirectDebit')
  AND    pp.bank_clearance_date IS NULL
  AND    pp.transaction_id      IS NOT NULL
  AND    pt.transaction_date    IS NOT NULL 
) ta ON (ta.transaction_id = pp.transaction_id)
WHEN MATCHED THEN
  UPDATE 
  SET pp.bank_clearance_date = ta.transaction_date,
      pp.effective_date      = ta.transaction_date,
      ta.receipt_date        = ta.transaction_date;

但是，如果你想更新同一行，那么你可以使用ROWID伪列（这实际上是一个指向该行的指针）：

MERGE INTO PAYMENT_PLAN pp
USING (
  SELECT pt.transaction_date,
         pp.ROWID AS rid
  FROM   payment_plan pp
         JOIN transactions pt
         ON (pp.transaction_id = pt.transaction_id)
         JOIN policy pol ON (PP.POLICY_ID = POL.POLICY_ID)
  WHERE  pp.payment_plan_status = 'CLOSED'
  AND    POL.PAYMENT_METHOD IN ('Cash','DirectDebit')
  AND    pp.bank_clearance_date IS NULL
  AND    pp.transaction_id      IS NOT NULL
  AND    pt.transaction_date    IS NOT NULL 
) ta ON (ta.rid = pp.ROWID)
WHEN MATCHED THEN
  UPDATE 
  SET pp.bank_clearance_date = ta.transaction_date,
      pp.effective_date      = ta.transaction_date,
      pp.receipt_date        = ta.transaction_date;

或者您可以将查询简化为：

MERGE INTO PAYMENT_PLAN pp
USING (
  SELECT pt.transaction_date,
         pt.transaction_id
  FROM   transactions pt
         ON (pp.transaction_id = pt.transaction_id)
         JOIN policy pol ON (PP.POLICY_ID = POL.POLICY_ID)
  WHERE  POL.PAYMENT_METHOD IN ('Cash','DirectDebit')
  AND    pt.transaction_date    IS NOT NULL
) ta ON (
    pp.payment_plan_status = 'CLOSED'
AND pp.bank_clearance_date IS NULL
AND pp.transaction_id      = ta.transaction_id
)
WHEN MATCHED THEN
  UPDATE 
  SET bank_clearance_date = ta.transaction_date,
      effective_date      = ta.transaction_date,
      receipt_date        = ta.transaction_date;