A developer has recently finished setting up an Application Load Balancer (ALB) for a group of EC2 instances in their AWS environment. As part of their configuration, they are responsible for ensuring that traffic is properly distributed across these instances through the ALB. However, upon initiating the testing phase for this configuration, the developer notices an operational oversight: the target groups, which are essential for managing and routing traffic to specific instances, have not been linked to the ALB.

In this scenario, what error code is the developer likely to encounter in their debug logs?