I'd verify that the username and password are correct (I think AEP8 requires some different password requirements so that may have changed).
I use Postman and have SSL certificate verification set to OFF, otherwise you'll want to exchange certs.
Here is some info from the POM Developers Guide
POM V4 Versioning and Supported Settings:
1. METHOD TYPE : GET/POST
2. Standard URL: Must be “https” nd use version v4 for 4.0.1 release https://<EP_Server_IP>/VP_POM_Service/v4/<resource-names>
Type: Basic Auth
Username: admin or org users (EP user having role to access the resource and specific operation)
X -Requested-With: RESTWeb (or rest)