How can solve Magento 2 Custom REST API returns Error?

Question

REST API returns Error:

{
    "messages": {
        "error": [
            {
                "code": 500,
                "message": "Fatal Error: 'Uncaught Error: Cannot instantiate interface MyCompany\\Slider\\Api\\SlidesRepositoryInterface in /var/www/html/magento.lan/vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php:50\nStack trace:\n#0 /var/www/html/magento.lan/vendor/magento/framework/ObjectManager/ObjectManager.php(70): Magento\\Framework\\ObjectManager\\Factory\\Dynamic\\Developer->create('MyCompany\\\\Adva...')\n#1 /var/www/html/magento.lan/vendor/magento/module-webapi/Controller/Rest/SynchronousRequestProcessor.php(90): Magento\\Framework\\ObjectManager\\ObjectManager->get('MyCompany\\\\Adva...')\n#2 /var/www/html/magento.lan/vendor/magento/module-webapi/Controller/Rest.php(188): Magento\\Webapi\\Controller\\Rest\\SynchronousRequestProcessor->process(Object(Magento\\Framework\\Webapi\\Rest\\Request\\Proxy))\n#3 /var/www/html/magento.lan/vendor/magento/framework/Interception/Interceptor.php(58): Magento\\Webapi\\Controller\\Rest->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#4 /var/www/html/magento.lan/vendor/magento/framework/Interc' in '/var/www/html/magento.lan/vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php' on line 50",
                "trace": "Trace is not available."
            }
        ]
    }
}

I also run all Cli :

rm -rf var/cache/* var/tmp/_cache var/composer_home/cache/* var/view_preprocessed/* var/page_cache/* var/report/* var/log/* generated/code/*  pub/static/frontend/* pub/static/adminhtml/* pub/static/_cache/*
php bin/magento setup:di:compile
php bin/magento s:up
php -d memory_limit=2G bin/magento index:reindex
php bin/magento c:f

cat /var/log/nginx/error.log

Stack trace:
#0 /var/www/html/magento.lan/vendor/magento/framework/ObjectManager/ObjectManager.php(70): Magento\Framework\ObjectManager\Factory\Dynamic\Developer->create('MyCompany\\Adva...')
#1 /var/www/html/magento.lan/vendor/magento/module-webapi/Controller/Rest/SynchronousRequestProcessor.php(90): Magento\Framework\ObjectManager\ObjectManager->get('MyCompany\\Adva...')
#2 /var/www/html/magento.lan/vendor/magento/module-webapi/Controller/Rest.php(188): Magento\Webapi\Controller\Rest\SynchronousRequestProcessor->process(Object(Magento\Framework\Webapi\Rest\Request\Proxy))
#3 /var/www/html/magento.lan/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Webapi\Controller\Rest->dispatch(Object(Magento\Framework\App\Request\Http))
#4 /var/www/html/magento" while reading response header from upstream, client: 127.0.0.1, server: www.magento.lan, request: "GET /rest/default/V1/apps/abanners HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.2-fpm.sock:", host: "www.magento.lan"

Answer 1

Usually the 500 error code its from the server, so as I see it its from PHP compiling.

The problem is with MyCompany\Slider\Api\SlidesRepositoryInterface , have you implemented it correctly, does it have in di.xml reference to it?

Vinai Kopp has a really nice explanation of it.

Magento 2 Repositories, Interfaces and the Web API