Ensure that the VPC Endpoint status is Available state

A VPC endpoint enables you to create a private connection between your VPC and another AWS service without requiring access over the Internet, through a NAT device, a VPN connection, or AWS Direct Connect. Instances in your VPC do not require public IP addresses to communicate with resources in the service. Traffic between your VPC and the other service does not leave the Amazon network. The endpoint connection must be in the Available state. If the endpoint connection is in the Pending or Rejected state, any connection sent to the Network Load Balancer from the interface endpoint times out.