Conditional Where Statement Involving a NULL value?
Use an OR to check if either the parameter matches the value, or both the parameter and the value are NULL, like this:

WHERE 
(
    [TBL_OUTBOUND_REVIEW].[ERROR_FREE] = @Status 
    OR 
    ([TBL_OUTBOUND_REVIEW].[ERROR_FREE] IS NULL AND @Status IS NULL)
)


You could also use a CASE statement as your IF if you really wanted to, but I think it's easier to read and understand the OR syntax above, so would prefer to use that.

WHERE 
(
    CASE WHEN @Status IS NULL AND [TBL_OUTBOUND_REVIEW].[ERROR_FREE] IS NULL THEN 1
       WHEN [TBL_OUTBOUND_REVIEW].[ERROR_FREE] = @Status THEN 1
       ELSE 0 END
) = 1