Lost trying to activate Office Professional 2021 on machine Frankensteined from an old one

Your title concerns Office Professional 2021, while the text also mentions Windows Pro. Those are separate products with separate activation rights, so first identify which application or Settings page reports the error.

For Office, a one-time purchase licenses one computer. An eligible Office 2021 license can be transferred to another computer you own under its transfer terms; the old installation alone does not invalidate it. Check your purchase documentation and license terms before assuming transferability.

If “already used” appears while redeeming the Office key, sign in to the Microsoft account originally associated with Office and install from Services & subscriptions. Redeeming that key again is unnecessary. Then activate the new installation; if it reports the activation limit, use Microsoft's telephone activation process and remove the prior copy as required. For a Windows activation error instead, check the installed edition and use the hardware-change Activation troubleshooter where eligible.