--4. Find Transaction Counter
IF @Return_Message <> ''
SELECT @Transaction_Counter = [Transaction Counter] FROM Accounts
WHERE [transaction date] = '@Date_Last_Payment' AND [policy number] = @policy_number AND [transaction type] IN ('new business', 'mta', 'renewal', 'Cancellation')
IF @Transaction_Counter = '' SET @Return_Message = @policy_number + ' failed, Transaction Counter not found, this was row number ' + @Row_Number
Change to
--4. Find Transaction Counter
IF @Return_Message <> ''
SELECT @Transaction_Counter = [Transaction Counter] FROM Accounts
WHERE [transaction date] = @Date_Last_Payment AND [policy number] = @policy_number AND [transaction type] IN ('new business', 'mta', 'renewal', 'Cancellation')
IF @Transaction_Counter = '' SET @Return_Message = @policy_number + ' failed, Transaction Counter not found, this was row number ' + @Row_Number
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden