Hi @Wunnam Soale
Please note, Read Aloud is only available for Office 2019, Office 2021, and Microsoft 365 Apps. Besides, Office 2016 Professional Plus key is only used to activate Office 2016. If it is used with other versions of Office, it may cause Office to transition to Office 2016 version.
According to your description, it's recommended to use the Office 2021 key to activate volume licensed versions of Office 2021, if you have a retail version of Office 2021 or Microsoft 365 apps, you may also use a licensed account to activate Office.
If there's anything I do not explain clearly, please feel free to let me know.