вы смешиваете имя таблицы и имя таблицы псевдонимов
JOIN Interfaces As Interface -- WHERE Interfaces.Availability = 2
если вы используете псевдоним, то используйте его везде
SELECT Nodes.Caption AS NodeName
,Interfaces.Caption AS InterfaceName
,AlertActive.Acknowledged
,AlertActive.AcknowledgedDateTime
,AlertActive.TriggeredDateTime
FROM Nodes
JOIN Interfaces As Interface ON Nodes.NodeID = Interface.NodeID
JOIN ActiveAlerts As Objects ON Objects.NodeID = Nodes.NodeID
AND Objects.NodeID = Interface.NodeID
JOIN AlertActiveObjects AS Alerts ON Alerts.AlertActiveID = Objects.AlertID
WHERE Interface.Availability = 2