Description
This error occurs when installing a plugin for wordpress.
Analysis
This may cause by incomplete configuration of wordpress，plugin installer couldn’t find related directories.
Fix
modify wp-config.php which in your wordpress root directory. Add following codes to the file.
define( 'FS_METHOD', 'direct');
define( 'FTP_BASE', '/your_absolutely_path_to_wordpress_root/' );
define( 'FTP_CONTENT_DIR', '/your_absolutely_path_to_wordpress_root/wp-content/' );
define( 'FTP_PLUGIN_DIR ', '/your_absolutely_path_to_wordpress_root/wp-content/plugins/' );
define( 'FTP_USER', 'your_ftp_username' );
define( 'FTP_PASS', 'your_ftp_password' );
define( 'FTP_HOST', 'your_ftp_domain_or_ip' );
define( 'FTP_SSL', false );