alter FUNCTION [Kuri].[fnGetAge](@kuri_cust_Id int,@amt decimal)
RETURNS SMALLINT
AS
    BEGIN
    DECLARE @isVallid  bit = 0
    declare @payed decimal(14,2)
    declare @totaltillnow decimal(14,2)
    select @payed = isnull(SUM(Payment.amt),0)  from Kuri.Payment where Payment.Kuri_Cust_ID = @kuri_Cust_id
    select @totaltillnow = isnull(SUM(NextLotAmount),0) from Kuri.Kuri_GivenDetails
    inner join Kuri.kuri_Customer  
    on Kuri_GivenDetails.kuri_Id  = kuri_Customer.kuri_ID 
     where kuri_Customer.kuri_Cust_id =  @kuri_Cust_id
     if((@payed + @amt) < @totaltillnow)
        set @isVallid = 1
        RETURN @isVallid
    END;
GO

ALTER TABLE [Kuri].[Payment]  WITH CHECK ADD  CONSTRAINT PaymentCheck CHECK (kuri.fnGetAge(kuri_Cust_ID,amt) >= 1 )
GO

error :

The ALTER TABLE statement conflicted with the CHECK constraint "PaymentCheck". The conflict occurred in database "MERP", table "Kuri.Payment".

Table structure is like this

CREATE TABLE [Kuri].[Payment](
    [payment_ID] [int] IDENTITY(1,1) NOT NULL,
    [payment_Date] [date] NOT NULL,
    [bill_No] [nvarchar](25) NOT NULL,
    [Kuri_Cust_ID] [int] NOT NULL,
    [vr_ID] [int] NOT NULL,
    [amt] [decimal](14, 2) NULL,
    [created_ID] [int] NULL,
    [created_Date] [datetime] NULL,
    [modified_ID] [int] NULL,
    [modified_Date] [datetime] NULL,
    [authorized_ID] [int] NULL,
    [authorized_Date] [datetime] NULL,
  CONSTRAINT [PK_Payment] PRIMARY KEY CLUSTERED 
  ([payment_ID] ASC)

 ALTER TABLE [Kuri].[Payment]  WITH CHECK ADD  CONSTRAINT [FK_Payment_kuri_Customer] FOREIGN KEY([Kuri_Cust_ID])
 REFERENCES [Kuri].[kuri_Customer] ([Kuri_Cust_ID])

 ALTER TABLE [Kuri].[Payment] CHECK CONSTRAINT [FK_Payment_kuri_Customer]