If I did not receive a 1099-B for sale of my Duke Energy stock on 9/24/19 what do I enter for the cost basis? The sale was for $9,626.40 but I do not know the cost basis.

we don't know the cost basis either.  contact the broker.  you should have received a 1099-B regardless of whether it shows the cost or not.  

Why didn't you get a 1099-B?  Your broker is REQUIRED to provide that.  But, depending on when you bought the stock, they may not be required to provide you your cost basis.  That's your responsibility to keep track of that.

Unknown cost basis

You have to report the sale on your tax return. Lacking any cost basis, the IRS will consider the entire sale amount as taxable. So, you need to make your best effort to determine the original cost basis, even if (worst case) it's a guess. Historical prices of publicly traded stocks are  readily available on the internet and should, at least, satisfy the IRS that your basis wasn't zero; just "google" “Historical Stock Prices". I use http://bigcharts.marketwatch.com/historical/. The stockholder relations dept. at Duke Energy may be able to help.