This error usually comes up when either the Livelink was not installed in the correct location or the Livelink could not initialise the socket properly. It also comes up when you attempt to export from Bridge but Unreal Engine is not running.
To solve this problem, first, make sure that both Bridge and Unreal Engine are running. If the issue still lingers, check if there is a green M button inside Unreal Engine.
If you don't see the button in Unreal Engine, it means the livelink was either not installed properly or was installed in the incorrect location.
To fix this, first, close Unreal Engine.
Then select one of your Downloaded assets in Bridge and checking if the Installation Folder was set to point to the correct location. It should be set to where your engine's plugins are installed, like this for instance: C:\Program Files\Epic Games\UE_4.22\Engine\Plugins. The path should basically be where your engine's plugins are installed.