1. A method of managing, via a server including a Graphical User Interface (GUI) and an external port, a service level agreement (SLA) including at least one SLA item with respect to an image forming apparatus connected to the server on a network, the method comprising: by the server: generating a GUI to receive a user input to register at least one of a plurality of SLA items of the image forming apparatus and to list the registered SLA items; setting a service management option with respect to a selected at least one of the registered SLA items listed in the generated GUI to measure a service level including error type information provided by the image forming apparatus to the server through the external port based on input by a user received at the generated GUI, wherein the setting the service management option with respect to the selected SLA item comprises setting one of: an automatic management mode in which the server automatically performs a predetermined event, the predetermined event corresponding to error information of an error management policy, regardless of a monitored service level of the image forming apparatus, a semiautomatic management mode in which the server automatically performs the predetermined event only when the monitored service level of the image forming apparatus is determined as less than a predetermined service level, and a manual management mode in which the server performs the predetermined event only after receiving another input by the user; monitoring the service level provided by the image forming apparatus of the selected SLA item and error information indicating a cause of an error upon the error occurring on the image forming apparatus; and performing the predetermined event based on the monitored service level, the error information, and the set service management mode, the method further comprising: adding a new SLA item to the plurality of SLA items by: obtaining SLA extended data, determining whether the GUI may be modified with respect to the plurality of SLA items, and installing the obtained SLA extended data on the server, upon determining the GUI may be modified.