Hello JackLin,
The AsEthIP library only implements communication as an EthernetIP Adapter (slave) device. Communication with the Cognex DM300X device will only function if it is the EthernetIP Scanner (master) device. If you want to use the B&R PLC as the EthernetIP Scanner, then you need to add an X20IF10D1-1 to act as the EthernetIP scanner on your system. Additional hardware is not required to act as a ModbusTCP Client (master) device.
There is an issue with your ST implementation (the same is in your LD), where you are using the wrong quotes to denote a STRING.
See the definition of STRING, WSTRING in the AS Help at: B&R Online Help (br-automation.com).