For the add standalone database operation, consider the following:
A machine used as a staging server can reside in the same domain as the machine hosting Veeam Explorer for Microsoft SQL Server or in a trusted domain. You cannot use a staging server that belongs to an untrusted domain to add a standalone database to Veeam Explorer for Microsoft SQL Server.
Does this mean that it cannot work without an AD configured?
Yes, it is by design. To use this feature, the staging server must be in the same domain or in a trusted domain as the machine that runs Veeam Explorer for Microsoft SQL Server. As a workaround, you can try running the staging server on the same server where Veeam Explorer is running.