Here are the most common issues you might experience with billing:
Please verify that firstname.lastname@example.org appears as a Billing Account User on your Google Billing Account. Refer to instructions to add the FireCloud 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 FireCloud Billing Project.
Another reason you may not see your Google Billing Account is that you may have enabled FireCloud 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 FireCloud Billing permissions for the Google ID under which you created your Google Billing Account. You may need to close your FireCloud session and/or log out of the incorrect Google account before re-enabling FireCloud Billing permissions.
This happens when you try to create a project but reached your Google quota. You may need to request additional projects from the Google Console as described in this Google Help article.
Google Billing Accounts set up using a free trial will automatically expire after 60 days unless you upgrade to a paid account. If your Google Billing Account expired, you will be unable to create a new FireCloud Billing Project. Log in to the Google Developers Console Billing page to verify that your Google Billing Account is active. You can select Show all accounts to display an expired Google Billing Account. If your Google Billing Account is "inactive" we suggest that you create a new Google Billing Account.
If you need further assistance or experience any other issues related to projects and billing, please comment directly on this article.