Access to SQL Syntax (Update query)

First of all: Format your code. This is the first step in solving syntax errors.

Then: Use always table alias names and prefix all columns with it. It's unclear to which source Operator and OperaterName belong.

The error is the missing END for the CASE expression. E.g.

UPDATE DSSM
SET    DSSM.EnteredBy = CASE WHEN SO.OperatorName IS NULL THEN SOMS.Operator
                             ELSE SO.OperatorName
                        END
FROM   dbo.view_Z_SalesOrderAmmendmentsSums SOMS
       INNER JOIN dbo.tbl_DriveSalesSidhilMatrix DSSM ON SOMS.Operator = DSSM.EnteredBy
       LEFT JOIN dbo.tbl_Syspro_Operators SO ON SOMS.Operator = SO.OperatorCode
WHERE  DSSM.RecordSource = N'Sidhil';

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But in this case (pun intended) we don't use CASE. Either the ANSI-SQL compatible COALESCE() or the T-SQL specific ISNULL() function. E.g.

UPDATE DSSM
SET    DSSM.EnteredBy = ISNULL(SO.OperatorName, SOMS.Operator)
FROM   dbo.view_Z_SalesOrderAmmendmentsSums SOMS
       INNER JOIN dbo.tbl_DriveSalesSidhilMatrix DSSM ON SOMS.Operator = DSSM.EnteredBy
       LEFT JOIN dbo.tbl_Syspro_Operators SO ON SOMS.Operator = SO.OperatorCode
WHERE  DSSM.RecordSource = N'Sidhil';

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p.s. embed you code into [code] tags (the CODE button) in all your posts, this increases readability and makes copying code easier.