SQL Server 2019

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CREATE TABLE [Proposals]
(
    [Proposal_Uid] [int] IDENTITY(1,1) NOT NULL,
    [Prime_Contract] [nvarchar](30) NULL,
    [Sub_Contract] [nvarchar](30) NULL,
    [Po_Id] [nvarchar](30) NULL,
    [Proposal_Title] [nvarchar](50) NULL,
    [Client_Name] [nvarchar](40) NULL,
    [Client_Code] [nvarchar](20) NULL,
    [Total_Proposal_Amount] [decimal](18, 2) NULL,
    [ODC_Amount] [decimal](18, 2) NULL,
    [Manager_Name] [nvarchar](30) NULL,
    [Admin] [nvarchar](30) NULL,
    [Due_Date] [date] NULL,
    [Start_Date] [date] NULL,
    [End_Date] [date] NULL,
    [Proposal_Number] [nvarchar](20) NULL,
    [Contract_Type] [nvarchar](16) NULL,

    CONSTRAINT [Proposal_Uid] 
        PRIMARY KEY CLUSTERED ([Proposal_Uid] ASC)
                    WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
                          IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
                          ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE PROCEDURE [Proposals_Create] 
    (@Prime_Contract nvarchar(30),
     @Sub_Contract nvarchar(30),
     @Po_Id nvarchar(30),
     @Proposal_Title nvarchar(50),
     @Client_Name nvarchar(40),
     @Client_Code nvarchar(20),
     @Total_Proposal_Amount decimal(18,2),
     @ODC_Amount decimal(18,2),
     @Manager_Name nvarchar(30),
     @Admin nvarchar(30),
     @Due_Date date,
     @Start_Date date,
     @End_Date date,
     @Proposal_Number nvarchar(20),
     @Contract_Type nvarchar(16),
     @NewId int output)
AS
BEGIN
    IF @NewId IS NULL
    BEGIN
        INSERT INTO Proposals (Prime_Contract, Sub_Contract, Po_Id, Proposal_Title, Client_Name, Client_Code,
                                           Total_Proposal_Amount, ODC_Amount, Manager_Name, Admin, Due_Date, Start_Date, End_Date, Proposal_Number, Contract_Type)
        VALUES (@Prime_Contract, @Sub_Contract, @Po_Id, @Proposal_Number, @Client_Name, @Client_Code,
                @Total_Proposal_Amount, @ODC_Amount, @Manager_Name, @Admin, @Due_Date, @Start_Date, @End_Date,
                @Proposal_Number, @Contract_Type)

        SET @NewId = SCOPE_IDENTITY()
    END

    RETURN @NewId
END
EXEC Proposals_Create 'hj','fd','fd','fd','fd','fd','0.23','1.24','fd','fd','2020/08/30','2020/08/30','2020/08/30','fd','fd';
Msg 201 Level 16 State 4 Line 0
Procedure or function 'Proposals_Create' expects parameter '@NewId', which was not supplied.

declare @test int;

EXEC Proposals_Create 'hj','fd','fd','fd','fd','fd','0.23','1.24','fd','fd',
                      '2020/08/30','2020/08/30','2020/08/30','fd','fd', @test OUTPUT;

select @test as 'new_identity';

set @test = null; -- because of the IF statement in your procedure...

EXEC Proposals_Create 'hj','fd','fd','fd','fd','fd','0.23','1.24','fd','fd',
                      '2020/08/30','2020/08/30','2020/08/30','fd','fd', @test OUTPUT;

select @test as 'new_identity';


new_identity


1




new_identity


2