Configuration of NLB Targets and API Integration Request when connect Rest API to VPC endpoint using VPC Link, NLB

I have a REST API in account A that I want to have access a private rest api in account b. In account A, I have configured the rest api with a vpc link. This VPC link is connected to my Network Load Balancer which should direct the request to the VPC Endpoint, still in account A. From the VPC Endpoint, the request should go over to the other account based on the resource policy crafted for the private rest api in Account b.

This architecture can be found in "Regional API Gateway to private API Gateway cross-account" section of https://aws.amazon.com/blogs/compute/architecture-patterns-for-consuming-private-apis-cross-account/

When running the API, from account A, I am getting a 500 error code that says " Execution failed due to configuration error: There was an internal error while executing your request"

I have the two following problems/questions:

1. When I check my registered targets in the assigned Target group for my NLB, it says the targets are unhealthy. I made sure to make sure these targets were IP addresses that were in the subnets of the NLB and VPC Endpoint CIDR Block.
2. I am using stageVariables.VpcLinkId to link the integration for the API, but I am unsure what I should have for the Endpoint URL, should it be the VPC endpoint DNS Name, the NLB DNS Name, or the private API DNS name (Account B). And should any of these be included in the header of the request from the api in Account A?