Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Do you know which Microsoft account was used to install Office? In that case, you should be able to use the Install option in the account; go to https://account.microsoft.com/services .
You may have to activate by phone. See the article at https://support.microsoft.com/en-us/office/-telephone-activation-is-no-longer-supported-for-your-product-error-when-activating-office-9b016cd2-0811-4cb3-b896-5a6a13177713 .