SQl query to update a table from another table using a where condition of not null for a field

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SQL

I have created a trigger to copy the records of a field from one table to another. I have got two tables namely School and SignOff where School has a primary key SchoolID and the signOff table has a foreign key for the field SchoolID from school. We have got multiple records in the SignOff table for the same SchoolID . I want to copy the field SchoolSignedOffDate from SignOff table to DateTravelPlanSubmitted in school table.

I want to copy field only if the Submitted field in the table SignOff is not null. Till now I have queried as

USE [TfLStarsDev]
GO
/****** Object:  Trigger [dbo].[Trigger1]    Script Date: 14/01/2016 09:50:10 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER TRIGGER [dbo].[Trigger1]
   ON  [dbo].[SignOff]
   AFTER UPDATE
AS
BEGIN TRY
    BEGIN TRANSACTION
	
	UPDATE School
  SET School.DateTravelPlanSubmitted = SchoolSignDate.SignoffSchoolDateMax
FROM School  INNER JOIN SignOff  ON  School.SchoolID = SignOff.SchoolID
inner join (select SignOff.SchoolID,SignoffSchoolDateMax=max(SignOff.SignedOffBySchoolDate) 
			from SignOff
			inner join School on School.SchoolID=SignOff.SchoolID
			group by SignOff.SchoolID) 
			SchoolSignDate on SchoolSignDate.SchoolID=School.SchoolID

where SignOff.Submitted IS NOT NULL

	 
    COMMIT TRAN 
END TRY
BEGIN CATCH
    IF @@TRANCOUNT > 0
        ROLLBACK TRAN 
END CATCH

I manged to copy without the where condition. Now the issue is only with the where condition. If I give as NOT NULL it is showing error.

Can anyone help to fix this issue.