error-in-the-execution-of-the-query-in-sql-server

My contract table :

My contractItem table:

My product table:

My sum function for set up the Total Amount at the contract table:

When I am using the "Insert Into "SQL to insert data into the contract data, there has an error occur:

I tried to use UPDATE sql also have the same error occur :

May you guys help me to resolve the problem?

Thanks for your help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!