This number is the number assigned by the State of South Carolina to the company or organization that paid you the dividends.
We in the Community have no way of knowing what that number might be. You will have to ask the company or organization that paid you the dividends and generated the 1099-DIV.
You might try entering the federal EIN (employer identification number) into box 15, if SC accepts it on e-filing (we can't know if it will).