Magento – Unable to access rest api : http://localhost/magento2/index.php/rest/V1/products

It seems to be a problem with the User Role.

To get started with the REST API in Magento 2 using token-based authentication, you will need to create a web service User Role and register that role to a new Magento 2 Admin User.

1. Login to the Magento 2 Admin Panel.
2. Go to System > User Roles and tap the Add New Role
3. Enter the Role Name.
4. In Your Password field, enter the current password of your Magento 2 Admin.
5. Now, on the left side, click Role Resources.
6. In the Resource Access, select only those that are required for your web service. (in your case, products should be selected)
7. Once done, hit the Save Role

Now, create a new user for the newly created role through these steps:

1. Go to System > All Users and hit the Add New User
2. Enter the required information including User Name, First and Last Name, Email, Password, etc.
3. Now, on the left side, click User Role and select the newly created Role.
4. Once done, click the Save User

Now, I will pass the newly created username and password with the API URL in the initial connection and receive the token. This token will be saved in a variable, which will be passed in the header for further calls.

<?php

//API URL for authentication
$apiURL="http://magento-91647-257956.cloudwaysapps.com/index.php/rest/V1/integration/admin/token";

//parameters passing with URL
$data = array("username" => "apiaccess", "password" => "cloudways123");
$data_string = json_encode($data);

$ch = curl_init($apiURL);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json","Content-Length: ".strlen($data_string)));
$token = curl_exec($ch);

//decoding generated token and saving it in a variable
$token=  json_decode($token); 

?>

You can fetch almost everything using Magento 2 REST API. In this answer, I will be fetching All Modules, but you can check out the List of REST APIs for Magento Open Source and see what you needed.

Now, I will pass the token (fetched earlier) in the header with the API URL to get all the modules installed on Magento 2 store.

<?php

//API URL for authentication
$apiURL="http://magento-91647-257956.cloudwaysapps.com/index.php/rest/V1/integration/admin/token";

//parameters passing with URL
$data = array("username" => "apiaccess", "password" => "cloudways123");
$data_string = json_encode($data);

$ch = curl_init($apiURL);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json","Content-Length: ".strlen($data_string)));
$token = curl_exec($ch);

//decoding generated token and saving it in a variable
$token=  json_decode($token);

//******************************************//

//Using above token into header
$headers = array("Authorization: Bearer ".$token);

//API URL to get all Magento 2 modules
$requestUrl='http://magento-91647-257956.cloudwaysapps.com/index.php/rest/V1/modules';

$ch = curl_init($requestUrl);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$result = curl_exec($ch);

//decoding result
$result=  json_decode($result);

//printing result
print_r($result);

?>

Once you execute the above code (with relevant information), run the file in the browser and you will see output similar to the following:

If you need further help, check this complete guide: https://www.cloudways.com/blog/magento-2-rest-api/