Unable to use forticlient on linux cli

I am new to use this software and I can login via a EMS on a WIndows device but I want to use it on another linux device with only command line interface.

I am using forticlient 6.4 on centos 7. According to the manual, I tried 

/opt/forticlient/epctrl

which returns

Starting epctrl... Created default epctrl configuration connect(127.0.0.1:45920): error Connection refused

So I tried 

/opt/forticlient/epctrl -u

But I don't know the password of this default EMS

Failed to connect to confighandler: connect(127.0.0.1:45920): error Connection refused Failed to unregister from EMS:

Also, there is no way to uninstall via yum as the software may have some way to protect itself.

What can I do to solve the issue? Thanks for your help.