i already solve it!
If you want the code to work, you have to add
more square brackets covering the return value in Config/system.phpbefore : return [ 'key'=>'blablabla' // other keys => values ]
after
return [ [ 'key'=>'blablabla' // other keys => values ] ] 'info' key in Config/system.phpbefore : 'key' => 'sales.carriers.raja_ongkir', 'name' => 'admin::app.configuration.index.sales.shipping-methods.raja-ongkir-shipping.page-title', 'sort' => 2, 'fields' => [ // field keys => values ]
after
'key' => 'sales.carriers.raja_ongkir', 'name' => 'admin::app.configuration.index.sales.shipping-methods.raja-ongkir-shipping.page-title', 'info' => 'admin::app.configuration.index.sales.shipping-methods.raja-ongkir-shipping.title-info', 'sort' => 2, 'fields' => [and dont forget to add the service provider in providers.php and add the namespaces to composer.json (from documentation)
"autoload": { ... "psr-4": { // Other PSR-4 namespaces "Webkul\\CustomShippingMethod\\": "packages/Webkul/CustomShippingMethod/src" } }