This applies to "Vlans Disabled: Use a single LAN configuration"
API call to getvlans for a MX single lan configured with an IP shows ['VLANs are not enabled for this network'].
The assumption is if getvlans can't see a configuration, then addvlan will fail as there is no 'id' or 'name' to reference.
There doesn't appear to be an API call for a single lan configuration scenario.
Where you at Meraki?