Exception class not found – PHP

I am trying to define a simple exception hierarchy:

Exception -> ServiceDenied -> IpBlocked

ServiceDenied.php:

<?php
namespace MorpherWs3Client;


class ServiceDenied extends Exception
{
    function __construct(string $message, int $code)
    {
        parent::__construct($message, $code);
    }
}

IpBlocked.php:

<?php
namespace MorpherWs3Client;

use MorpherWs3ClientServiceDenied; // this line is greyed out by the IDE

class IpBlocked extends ServiceDenied // this line has the error
{
    function __construct(string $message, int $code)
    {
        parent::__construct($message, $code);
    }
}

Both files are in the same folder.

Now when I run unit tests, I get the following error:

Error : Class 'MorpherWs3ClientServiceDenied' not found
 C:Codemorpher-ws3-php-clientsrcexceptionsIpBlocked.php:6
 C:Codemorpher-ws3-php-clientvendorcomposerClassLoader.php:571
 C:Codemorpher-ws3-php-clientvendorcomposerClassLoader.php:428

Why am I getting this error and how do I fix it?

Removing the greyed out line does not have any effect. It still says "Class ‘MorpherWs3ClientServiceDenied’ not found".

Sorry if it’s something obvious. I’m new to PHP.