A family of Microsoft word processing software products for creating web, email, and print documents.
If you are paying for a subscription, you don't need a product key, and in fact you can't (re-)use any product keys that you have used in the past. You install and activate Office using your account at https://account.microsoft.com/services.
If you are saying that you do not have an Office product, the cheapest option is to get a one-time payment license such as Microsoft Office Home 2024. It is for one user and one computer (PC or Mac). See https://www.microsoft.com/en-us/microsoft-365/p/office-home-2024/cfq7ttc0pqvj.