Sync Business license can be applied only on workstations. If applied on a Server OS - any Windows Server and Linux with non-arm CPU (NAS also count) - license will give the warning.
Actually, license will get applied, but sharing files and folders will be stopped, linking devices will not work, etc.
You can purchase Sync Business license with Server support from the purchase page. Currently, server support feature is available for packages with 20 seats or more.
If you've already bought Sync license and only later realized that you need server support, you can login to the License Management Console and increase the seats number to the minimal required for the server support feature to become available.