Please verify that firstname.lastname@example.org appears as a Billing Account User on your Google Billing Account. Refer to instructions to add the Terra Billing Account User to your Google Billing Account in this article. After you add the Billing Account User, you should be able to create a Terra Billing Project.
Another reason you may not see your Google Billing Account is that you may have enabled Terra Billing permissions for the incorrect Google ID (e.g. Gmail address). This occasionally happens if you have both a professional Google account and a personal Google account, and you happen to be signed into your personal account. Please be sure to enable Terra Billing permissions for the Google ID under which you created your Google Billing Account. You may need to close your Terra session and/or log out of the incorrect Google account before re-enabling Terra Billing permissions.