There will be two different manager entries.
The one for version 2 called “isymphony”. This is created when the isymphony module (iSymphony) is installed in FreePBX. If you did not have the Asterisk API/Manager Users module installed when the isymphony module was installed you will hot have the isymphony manager entry. You will have to reinstall the isymphony module from Admin->Module Admin and then Apply Config. This should get version 2 working.
The second one is for version 3 called “cxpanel”. This is created when an apply config is executed in FreePBX and you have the cxpanel module (iSymphony V3) installed. The error message you are getting in Admin->iSymphony V3 is indicative of the fact that you may not have the iSymphony V3 server installed. Installing the iSymphony V3 module will not install the server. See the documentation at docs.getisymphony.com for instructions on how to install the V3 server portion.