If Excel is already installed but you've lost your documentation, you can try using the to retrieve the last five digits of the key to help identify it: Open CMD as an Administrator.
If you bought a retail box, the key is usually on a card inside the sleeve.
Use the official Microsoft Support site if your genuine key is being rejected.
Check the inbox used during purchase for a message from the Microsoft Store or a third-party retailer.
If you need Excel 2019 but don't want a subscription, here are legitimate, safe options: