What is responsible for this error when connecting to LAN (The connection request to 127.0.0.1:26500 failed. Are you sure the server can be connected to?)

I was testing my game for bugs so i tried connecting to a server that did not exist .

needless to say i got an error stating "`The connection request to 127.0.0.1:26500 failed. Are you sure the server can be connected to?`"

There is a check that is performed when you connect to a network . it determines is the network can be connected to .

What is that check ?

I have tried to use many methods to find it but with no luck.