If I try to install a plugin the request to http:///mixer/install.json fails:
{
"message": "Method Not Allowed",
"url": "/mixer/install.json",
"code": 405,
"file": "C:\*\vendor\cakephp\cakephp\src\Http\ServerRequest.php",
"line": 1758
}

System:
Windows 10
Apache/PHP 7