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 none of the Pro features will work - sharing files and advanced folders will be stopped, linking devices will not work, etc.
You can purchase Sync Business license with Server support form the purchase page. Enter the number of seats that will need server support. It cannot be bigger that the total number of seats in the license.
If you've already bought Sync license and only later realized that you need server support, login to the License Management Console and add the server seats there.