I follow these steps and got the problem solved.
https://support.inductiveautomation.com/index.php?/Knowledgebase/Article/View/4/2/setting-up-and-troubleshooting-opc-com-connections
the problem is caused by Ignition could not find the right COM object to create. so, install OPC Core component and then reboot the machine, should resolve the problem.
My machine is 64 bit windows, choose x64. during the intallatin process, it might come up with a question, telling you that some process need to stop before proceeding, like OpcEnum, I just kill that process and click cont. (beware, do this at your own risk)
