There are a couple of reasons that Swarm Failed to kick off.
1.>Some of the more common ones are that the firewall or antivirus may be blocking it from starting properly. You can disable these and then try to build lighting to see if that works.
2.>The other is that there may be multiple swarm agents opened in your take bar (next to your computer clock in the bottom right). When swarm agent is initialized there will be a orange and black S icon that appears. Close any your see here and then try to build lighting.
3.>If none of that works, can you try building lighting in a new template project like the first person or third person templates? This can rule out if it’s a project specific problem or swarm agent issue possibly.