Magento Cannot instantiate interface

Question

I wrote a custom api for magento 2.3.1

When i call the endpoint that i was created i get the following error

{
"messages": {
    "error": [
        {
            "code": 500,
            "message": "Server internal error. See details in report api/1560842508114"
        }
    ]
}
}

Report detail:

"Fatal Error: 'Uncaught Error: Cannot instantiate interface Demo\Hello\Api\HelloInterface in /var/www/html/vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php:116\nStack trace:\n#0 /var/www/html/vendor/magento/framework/ObjectManager/Factory/Compiled.php(108): Magento\Framework\ObjectManager\Factory\AbstractFactory->createObject('Demo\\Hello\\Api\\...', Array)\n#1 /var/www/html/vendor/magento/framework/ObjectManager/ObjectManager.php(70): Magento\Framework\ObjectManager\Factory\Compiled->create('Demo\\Hello\\Api\\...')\n#2 /var/www/html/vendor/magento/module-webapi/Controller/Rest/SynchronousRequestProcessor.php(90): Magento\Framework\ObjectManager\ObjectManager->get('Demo\\Hello\\Api\\...')\n#3 /var/www/html/vendor/magento/module-webapi/Controller/Rest.php(188): Magento\Webapi\Controller\Rest\SynchronousRequestProcessor->process(Object(Magento\Framework\Webapi\Rest\Request\Proxy))\n#4 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Webapi\Controller\Rest->dispatch(Object(Magento\Fr' in '/var/www/html/vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php' on line 116"

app/code/Demo/Hello/registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::MODULE,
'Demo_Hello',
__DIR__
);

app/code/Demo/Hello/Api/HelloInterface.php

<?php
namespace Demo\Hello\Api;
interface HelloInterface
{
   /**
   * Returns greeting message to user
   *
   * @api
   * @param string $name Users name.
   * @return string Greeting message with users name.
   */
   public function name($name);
}

app/code/Demo/Hello/etc/di.xml

    <?xml version="1.0"?>
    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Demo\Hello\Api\HelloInterface"
                type="Demo\Hello\Model\Hello" />
</config>

app/code/Demo/Hello/Model/Hello.php

<?php
namespace Demo\Hello\Model;
use Demo\Hello\Api\HelloInterface;
class Hello implements HelloInterface
{
   /**
   * Returns greeting message to user
   *
   * @api
   * @param string $name Users name.
   * @return string Greeting message with users name.
   */
   public function name($name) {
      return "Hello, " . $name;
   }
}

Answer 1

I would suggest trying to use swagger to execute your API. Also, please refer below links that will help you find out the issue in your current code.

https://inchoo.net/magento-2/magento-2-custom-api/
https://www.scommerce-mage.com/blog/magento-2-how-to-create-custom-api.html

Hope this will help you.