Dear Jan,
We believe you may not be able to install the Adobe Sign SharePoint Online application due to the assigned permission rights within SharePoint.
As a SharePoint site collection administrator, you'd require approval from the SharePoint Global Admin (sometimes referred to as a tenant admin or super admin) to download an app in SharePoint. In this case, when a site collection admin tries to download or install an Add-In, a request is initiated for the super admin to approve and until then, the status shows up as ''Pending Purchase.'' This is true when installing any add-in in SharePoint and not just Adobe Sign.
For step-by-step instructions for the Super Admin on how to either approve an App download request, or how to assign the necessary permissions to you to download applications in SharePoint, please go to: https://docs.microsoft.com/en-us/sharepoint/request-app-installation-permissions
We hope to have helped you get this resolved. If you find that you have any more questions, please don't hesitate to get back in touch.
Kind regards,