Cloudback handles paid plans using GitHub Marketplace. As for now, the marketplace is restricted for GitHub invoiced customers. If you have a GitHub Enterprise account and you pay by invoice, you are not able to purchase paid plans using the marketplace. GitHub will show you the error message
Unfortunately, invoiced customers cannot purchase paid plans on the GitHub Marketplace..
We support direct payments to Cloudback using bank transfers. Please contact us and we will arrange a free trial period for you.