In ServiceNow Discovery, when encountering the error 'Active, couldn't classify' during the discovery of a new SNMP network device, it typically indicates an issue with the classification process. Classification is a crucial initial step in the Discovery process, where the system determines the type of device it is interacting with. If an SNMP Classification rule for the new device type does not exist or needs updating, Discovery will be unable to classify and thus properly discover the device. Incorrect SNMP credentials or firewall issues usually result in different types of errors, such as authentication failures or no response from the device.