If you need it for your development environment you can enable settings.local.php in your settings.php

if (file_exists($app_root . '/' . $site_path . '/settings.local.php')) {
  include $app_root . '/' . $site_path . '/settings.local.php';
}

and than make sure ONLY in your local settings.local.php (not on your live enviroment) the following settings are used

$config['system.performance']['css']['preprocess'] = FALSE;
$config['system.performance']['js']['preprocess'] = FALSE;

If you need it for your development environment you can enable settings.local.php in your settings.php

if (file_exists($app_root . '/' . $site_path . '/settings.local.php')) {
  include $app_root . '/' . $site_path . '/settings.local.php';
}

and than make sure ONLY in your local settings.local.php (not on your live enviroment) the following settings are used

$config['system.performance']['css']['preprocess'] = FALSE;
$config['system.performance']['js']['preprocess'] = FALSE;

NOTE: those settings can probably be found in sites/default.settings.local.php which you can copy to sites/default/settings.local.php and make use of.