You've asked this three times now.
You DO NOT hold shares in Yahoo Portfolio. It's NOT a brokerage. Just sign into your actual brokerage account and sell your shares.
Yes you "can" in theory somehow link your brokerage account to Yahoo so that it forwards your sell order to your brokerage, but why would you do that?
If you have multiple brokerage accounts, just use a spreadsheet to keep track of everything like a normal person.
You said you were old, not slow. Then keep up, cupcake. Yahoo IS a joke so I have no clue why you are trying to sell stocks through Yahoo Portfolio.
Fellow old person who would never, ever in a million years rely on Yahoo to execute or keep track of any kind of financial transaction.