Be cautious of websites selling "Office 2021 LTSC Keys" for $10–$20. These are often:

For legitimate Office 2021 LTSC keys, deployment requires technical infrastructure that differs from standard Microsoft 365 deployment.

Activating Office 2021 LTSC requires specific keys based on your organization's infrastructure. Microsoft provides three primary methods through Microsoft Learn : Overview of Office LTSC 2021 - Microsoft Learn