Must declare the scalar variable in stored procedure in SQL Server - sql-server

Question

Could anyone have idea why my stored procedure encountered an error when I executed it?
I have tried to isolate in a simple select statement and supplying the value in where clause, it works fine. But when I put to stored procedure with a declared parameter, it won't work anymore.
This is the error after I execute the stored procedure:
Msg 137, Level 15, State 2, Line 30
Must declare the scalar variable "#BU".
See below for my stored procedure.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[sp_Planning_GetMonthlyPivotX]
#BU char(7)
AS
BEGIN
DECLARE #PivotColumns AS NVARCHAR(MAX)
SELECT #PivotColumns = COALESCE(#PivotColumns + ',', '') + QUOTENAME([YEARMONTH])
FROM
(SELECT DISTINCT
CONVERT(CHAR(6), CAST([releaseddate] AS DATE), 112) AS [YEARMONTH]
FROM
[dbo].[ShopOrder]) AS PivotQuery
ORDER BY
[YEARMONTH]
DECLARE #query NVARCHAR(MAX)
SET #query = 'SELECT *
FROM
(SELECT
a.[ItemCode], b.[Description],
B.[Catalog] as [BU], a.[WH],
f.[CustNo], f.[CustName],
b.[IsOutSource],
[dbo].[f_Routings_GetMCHr](a.[ItemCode]) as [Std.Time],
d.[Tonnage], d.[MachineNo],
d.[MachineNo] as [FixedMachine],
[dbo].[f_BOM_GetChildItem](a.ItemCode) as [ChildItem],
[dbo].[f_Item_GetItemDescription]([dbo].[f_BOM_GetChildItem](a.ItemCode)) as [ChildDescription],
[dbo].[f_BOM_GetChildQty](a.ItemCode,[dbo].[f_BOM_GetChildItem](a.ItemCode)) as [Usage],
e.[TRP],
CONVERT(char(6), cast([ReleasedDate] as date),112) as [YearMonthRel],
[dbo].[f_Inventory_GetTotalStock](b.[ItemCode]) as [Inventory],
b.[MinStockLvl] as [SafetyStock],
a.[RequiredQty] as [ShopOrderQty],
a.[FinishedQty] as [ActualQty],
g.[WorkTimeHr] as [AvailableTime]
FROM
[CPS].[dbo].[ShopOrder] AS a with(nolock)
LEFT OUTER JOIN
[CPS].[dbo].[items] AS b with(nolock) ON a.[ItemCode] = b.[ItemCode]
LEFT OUTER JOIN
[CPS].[dbo].[ItemByMachine] AS d with(nolock) ON a.[ItemCode] = d.[ItemCode]
LEFT OUTER JOIN
[CPS].[dbo].[MCTonnage] as e with(nolock) ON d.[Tonnage] = e.[Tonnage]
LEFT OUTER JOIN
[CPS].[dbo].[Customer] as f with(nolock) ON b.[CustNo] = f.[CustNo]
LEFT OUTER JOIN
[CPS].[dbo].[Machine] as g with(nolock) ON d.[MachineNo] = g.[MachineNo]
WHERE
b.[Catalog] = #BU) AS T
PIVOT
(
SUM([ShopOrderQty])
FOR [YearMonthRel] IN (' + #PivotColumns + ')
) p order by [Tonnage],[MachineNo],[ItemCode];'
Execute SP_EXECUTESQL #query
END

Answer 1

should be
WHERE b.[Catalog] = ''' + #BU + '''
and your select should be
SET #query = N'SELECT * FROM
as nvarchar