How to use subquery in Insert statement in SQL Server

I have executed it and i have tested the stored procedure once again

Tables

Customers

CREATE TABLE [dbo].[Customers](
	[CustomerId] [int] IDENTITY(1,1) NOT NULL,
	[Name] [char](100) NOT NULL,
	[Country] [char](50) NOT NULL
) ON [PRIMARY]

GO

tblSales

CREATE TABLE [dbo].[tblSales](
	[Net] [int] NULL,
	[Vat] [int] NULL,
	[TaxBase] [varchar](50) NULL,
	[Change] [varchar](50) NULL,
	[CustID] [varchar](10) NULL
) ON [PRIMARY]

GO

Customers records

CustomerId   Name                 Country 1                John Hammond       United States 2                Mudassar  Khan     USA 3                Suzanne  Mathews   France 4                Robert Schidner    Germany

 Stored Procedure

-- InsertSales 1,4,'test','test','Robert Schidner'
CREATE PROCEDURE InsertSales
    @Net INT
    ,@VAT INT
    ,@TaxBase VARCHAR(50)
    ,@Change VARCHAR(50)
    ,@FirstName VARCHAR(50)
AS
BEGIN  
    SET NOCOUNT ON;
     
    DECLARE @CustID VARCHAR(10)
    SET @CustID = (Select CustomerId FROM Customers WHERE Name = @FirstName)   
    INSERT INTO tblSales(Net,VAT,TaxBase,Change,CustID)
    VALUES(@Net,@VAT,@TaxBase,@Change,@CustID)
END
GO

 Executing the stored procedure

EXEC [dbo].[InsertSales]
@Net = 3,
@VAT = 4,
@TaxBase = N'test1',
@Change = N'test2',
@FirstName = N'Suzanne  Mathews'

Inserted records

Net    Vat    TaxBase    Change    CustID 3        4        test1       test2        3

You need to use VARCHAR (50) Data type. Problem is that you have defined Table Column data type as text null so you need to change it to VARCHAR (no of characters) data type.