Если вы просто хотите избавиться от ошибки, но не думаете, что она даст вам правильный вывод.
AS
BEGIN
SELECT
p.[ProductID]
,l1.[Description] AS 'Category'
,l2.[Description] AS 'Supplier Name'
,p.[PurchasePrice] AS 'Purchase Price'
,p.[SalesPrice] AS 'Sales Price'
,l3.[Description] AS 'Sizes'
from [dbo].[Productss] p
INNER JOIN [dbo].[ListTypesData] l1 ON p.CategoryID) = l1.ListTypeDataID
INNER JOIN [dbo].[ListTypesData] l2 ON Convert(varchar(max),p.SupplierName) = Convert(varchar(max),l2.ListTypeDataID)
INNER JOIN [dbo].[ProductSizes] ps ON p.ProductID = ps.[ProductID]
INNER JOIN [dbo].[ListTypesData] l3 ON ps.[SizeID] = l3.ListTypeDataID
END