Added application load balancer event but getting HTTP 503: Service Unavailable errors

HI folks. I’ve added an alb event in my serverless.yml

functions:
      events:
        - alb:
          listenerArn: !Ref ALBListenerId
          priority: 1
          conditions:
            path: /path/url
            method:
              - POST
resources:
    Resources:
      SDKALBListener:
        Type: AWS::ElasticLoadBalancingV2::Listener
        Properties:
          DefaultActions:
            -
              Order: 1
              Type: forward
              TargetGroupArn: !ImportValue ALBTargetGroupArn
          LoadBalancerArn: !ImportValue ALBArn
          Port: 80
          Protocol: HTTP

With the ALBTargetGroupArn and ALBArn being exported from a different stack. However when I try hit the alb url with matching path I get a 503 error. This generally means the target groups for the load balancer have no registered targets. What could be the cause of this as I’ve clearly added triggers in my serverless.yml? Also noticed that after I deploy the alb trigger is not visible on the lambda dashboard. If I add manually everything works ok