macOS network extension fail to load on method of startVPNTunnel

Question

I'm writing some VPN agent, and in order to connect with my remote vpn server I run the following command after setting the connection object (from type NEVPNConnection)

try tunnel.connection.startVPNTunnel(options: [:] as [String : NSObject])

It looks like the connection establishment is failed, and from the logs I see the following relevant messages :

2021-05-10 08:43:00.621853+0300 0x6924e    Error       0x6cbc0              678    0    
neagent: [com.apple.networkextension:] NEAgentSession: failed to create the delegate

...

2021-05-10 08:43:00.622039+0300 0x693d2    Error       0x6cbc0              251    0
    nesessionmanager: [com.apple.networkextension:] com.company.vpn-client-mac[678]: Tearing down XPC 
connection due to setup error: Error Domain=NEAgentErrorDomain Code=2 "(null)"

I've made sure that the network extension sits in the bundle of my application under subfolder Plugins, but I cannot understand from the logs why did it fail to load the extension.

Any idea how to debug this ?