Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
If the key already belongs to an account, use that account for installation.
If the key has not been added to a Microsoft account, do that at https://setup.office.com . You will be asked to sign in first. Choose the account you want to use with your Office product. Then enter the key.
You should be able to install from the associated account at https://account.microsoft.com/services .
For more on activation, see https://support.microsoft.com/en-us/windows/activation/activate-microsoft-perpetual-products-using-the-product-activation-portal.