Error in using SELECT to set a variable value in SQL - sql

Question

I have following stored procedure and I have identified the issue with the stored procedure is that using select to set the value of #DeliveryAddress variable and so for all the Letter Requests raised it is only retaining the last value which is in Requests. I tried using SET by explicitly setting each value used in #DeliveryAddresss Calculation but of no use as I ended of getting error that subquery returns multiple rows.
I am unable to understand what shall I change in this SP, so that for each value in #Requests, a different #deliveryAddress value is set and used for insertion in LetterRequest table. Please help.
Note #WorkFlowAcct is a temp table having around 10 unique AccountIds and for each account we have atleast one debtorid.
ALTER PROCEDURE [dbo].[WorkFlow_Action_RequestLetterPref]
AS
DECLARE #LetterID INTEGER;
DECLARE #LetterType CHAR(3);
DECLARE #LetterDescription VARCHAR(50);
DECLARE #JobName VARCHAR(256);
DECLARE #DateCreated DATETIME;
DECLARE #DeliveryMethod VARCHAR(7);
DECLARE #DeliveryAddress VARCHAR(1023);
SELECT #LetterID = [LetterID],
#LetterType = CASE
WHEN [type] IN ('SIF', 'PIF', 'PPS', 'PDC', 'ATT', 'CUS') THEN [type]
ELSE 'DUN'
END,
#LetterDescription = ISNULL([Description], ''),
FROM [dbo].[letter]
WHERE [code] = #LetterCode;
DECLARE #Requests TABLE (
[AccountID] INTEGER NOT NULL,
[DebtorID] INTEGER NOT NULL,
[Seq] INTEGER NOT NULL,
[ErrorMessage] VARCHAR(500) NULL
);
IF #PrimaryDebtor = 1 OR #LetterType IN ('CUS', 'ATT') BEGIN
INSERT INTO #Requests ([AccountID], [DebtorID], [Seq])
SELECT DISTINCT [master].[number] AS [AccountID],
[Debtors].[DebtorID] AS [DebtorID],
[Debtors].[Seq] AS [Seq],
FROM #WorkFlowAcct AS [WorkFlowAcct]
INNER JOIN [dbo].[master] WITH (NOLOCK)
ON [WorkFlowAcct].[AccountID] = [master].[number]
INNER JOIN [dbo].[customer] WITH (NOLOCK)
ON [master].[customer] = [customer].[customer]
INNER JOIN [dbo].[Debtors] WITH (NOLOCK)
ON [master].[number] = [Debtors].[number]
AND [Debtors].[Seq] = [master].[PSeq]
LEFT OUTER JOIN [dbo].[DebtorAttorneys]
ON [Debtors].[DebtorID] = [DebtorAttorneys].[DebtorID];
END;
DECLARE #Street1 VARCHAR(512);
DECLARE #Street2 VARCHAR(512);
DECLARE #City VARCHAR(512);
DECLARE #Country VARCHAR(512);
DECLARE #Zip VARCHAR(512);
IF #Pref = 'Letter'
BEGIN
SET #DeliveryMethod = 'Letter';
SELECT #DeliveryAddress = [Street1] + ' ' + [Street2] + ' ' + [City] + ' ' + [Zipcode] + ' ' + [Country], #Street1 = [Street1], #Street2 = [Street2], #City = [City], #Country = [Country], #Zip = [ZipCode] FROM [Debtors] inner join #Requests AS Requests on [Debtors].[DebtorID] = Requests.[DebtorID] AND [Street1] IS NOT NULL;
IF #Street2 IS NULL
BEGIN
SET #DeliveryAddress = #Street1 + ' ' + #City + ' ' + #Zip + ' ' + #Country;
END
END
SET #DateCreated = GETDATE();
SET #JobName = 'WorkFlow_' + CAST(NEWID() AS CHAR(36)) + CAST(NEWID() AS CHAR(36)) + CAST(NEWID() AS CHAR(36)) + CONVERT(VARCHAR(50), GETDATE(), 126);
BEGIN TRANSACTION;
--Updated params for current Letter Request Table
INSERT INTO [dbo].[LetterRequest] ([AccountID], [LetterID], [LetterCode], [DeliveryMethod], [DeliveryAddress])
SELECT [Requests].[AccountID],
[Requests].[CustomerCode],
#LetterID AS [LetterID],
#LetterCode AS [LetterCode],
#DeliveryMethod AS [DeliveryMethod],
#DeliveryAddress AS [DeliveryAddress]
FROM #Requests AS [Requests]
INNER JOIN #AllowedCustomers AS [AllowedCustomers]
ON [Requests].[CustomerCode] = [AllowedCustomers].[CustomerCode]
WHERE [Requests].[ErrorMessage] IS NULL;
COMMIT TRANSACTION;
RETURN 0;