Procedure or function !!! has too many arguments specified

Use the following command before defining them:

cmd.Parameters.Clear()

You invoke the function with 2 parameters (@GenId and @Description):

EXEC etl.etl_M_Update_Promo @GenID, @Description

However you have declared the function to take 1 argument:

ALTER PROCEDURE [etl].[etl_M_Update_Promo]
    @GenId bigint = 0

SQL Server is telling you that [etl_M_Update_Promo] only takes 1 parameter (@GenId)

You can alter the procedure to take two parameters by specifying @Description.

ALTER PROCEDURE [etl].[etl_M_Update_Promo]
    @GenId bigint = 0,
    @Description NVARCHAR(50)
AS 

.... Rest of your code.