Cannot use WHERE variable in where clause for Insert stored procedure

Question

I'm constructing a stored procedure for Insert only. What rows I want to insert to my target table depend on a composite key of fields. I'm trying to construct it before and then use it in the WHERE-clause, but I get this error:

An expression of non-boolean type specified in a context where a condition is expected, near ')'*

My @WhereClause is holding this value from the string split: [Column1] = S.[Column1] AND [Column2] = S.[Column2] AND [Column3] = S.[Column3]

This is my code

AS

BEGIN

    SET XACT_ABORT ON

    DECLARE @compositeKey NVARCHAR(MAX) =   'Column1,Column2,Column3';
    DECLARE @WhereClause NVARCHAR(MAX) = '';

    BEGIN TRY

        BEGIN TRANSACTION

        SELECT @WhereClause = STRING_AGG(QUOTENAME(Value) + ' = S.' + QUOTENAME(Value), ' AND ')
        FROM STRING_SPLIT(@compositeKey, ',');
        PRINT @WhereClause;

        INSERT INTO [target].[targetTable]
        (
        Bunch of columns...
        )
        SELECT
        Bunch of columns...
        FROM [source].[sourceTable] AS S
        WHERE NOT EXISTS (
            SELECT 1 FROM [target].[targetTable]
            WHERE + @WhereClause
        );
      
        COMMIT TRANSACTION
   
    END TRY
    BEGIN CATCH

        IF @@trancount > 0 ROLLBACK TRANSACTION
        DECLARE @msg nvarchar(2048) = error_message()  
        RAISERROR (@msg, 16, 1)
        RETURN 55555

    END CATCH

END

It works if I put the result from my stringsplit directly behind the Where, that's why I thought I would work with the variable.