Reference Material:

https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html
Permissions to invoke the Lambda function
If you create the target group and register the Lambda function using the AWS Management Console, the console adds the required permissions to your Lambda function policy on your behalf. Otherwise, after you create the target group and register the function using the AWS CLI, you must use the add-permission command to grant Elastic Load Balancing permission to invoke your Lambda function. We recommend that you use the aws:SourceAccount and aws:SourceArn condition keys to restrict function invocation to the specified target group.
aws lambda add-permission
--function-name lambda-function-arn-with-alias-name
--statement-id elb1
--principal elasticloadbalancing.amazonaws.com
....