Custom Endpoints not working

You’re using the wrong route. The first two arguments of register_rest_route() ar the namespace, and the route. The URL to a REST API endpoint is:

(your domain) + '/wp-json/' + (namespace) + "https://wordpress.stackexchange.com/" + (route)

So the full URL to your endpoint is:

http://domain.com/wp-json/knpv-get/get-submissions

But you’re attempting to view:

http://domain.com/wp-json/knpv-get

Which won’t return the response from your callback function. However if you look closely at the result, you’ll see that it’s giving you a list of routes under the knpv-get namespace:

{
  "namespace": "knpv-get",
  "routes": {
    "\/knpv-get": {
      "namespace": "knpv-get",
      "methods": [
        "GET"
      ],
      "endpoints": [
        {
          "methods": [
            "GET"
          ],
          "args": {
            "namespace": {
              "required": false,
              "default": "knpv-get"
            },
            "context": {
              "required": false,
              "default": "view"
            }
          }
        }
      ],
      "_links": {
        "self": "http:\/\/knoppysdev.com\/vendor-portal\/wp-json\/knpv-get"
      }
    },
    "\/knpv-get\/get-submissions": {
      "namespace": "knpv-get",
      "methods": [
        "GET"
      ],
      "endpoints": [
        {
          "methods": [
            "GET"
          ],
          "args": [

          ]
        }
      ],
      "_links": {
        "self": "http:\/\/knoppysdev.com\/vendor-portal\/wp-json\/knpv-get\/get-submissions"
      }
    }
  },
  "_links": {
    "up": [
      {
        "href": "http:\/\/knoppysdev.com\/vendor-portal\/wp-json\/"
      }
    ]
  }
}