Error message in Cloud Manager: “The domain is already used by another Tenant."

This error indicates the .onmicrosoft.com domain provided has not authorized TechSoup as a partner in the Microsoft 365 Admin Center yet. This is a required step.
 

How to Authorize TechSoup as Your Provider
 

Complete this authorization by visiting the Partner Relationships page in your Microsoft 365 Admin Center.

You’ll need an account global administrator’s login to see this Partner Relationships page. The person who registered your organization on Microsoft for Nonprofits is, by default, a global administrator.

You can also find the Partner Relationships page by visiting your Microsoft 365 Admin Center and, in the menu on the left of the page, choosing Show All > Settings > Partner relationships. On that page you’ll see all of the partners you have authorized as providers.

Once you successfully authorize TechSoup as a partner, you should be able to proceed with checkout.