Can't activate tape gateway using a VPC Endpoint

I've been bashing my head on this for over a week. The ticket I have open on my Developer Support isn't helping.

My supervisor wants to start mirroring backups to AWS using a virtual tape library. I have the gateway vm deployed to our VMware environment. If I try to activate using a public endpoint, all is well. When I try to activate using a VPC endpoint, my browser returns "Internal Error".

I've gone over this document a dozen times: https://docs.aws.amazon.com/storagegateway/latest/userguide/gateway-private-link.html

My VPC is up, the endpoint is configured per the document, the Security Group has inbound rules allowing the necessary ports ... but the blasted thing just. will. not. activate.

Are we barking up the wrong tree here? I'm not even sure we need to be using a VPC here.

Any direction is appreciated. This is my first experience with AWS.