Myujth

When a woocommerce order is created the status of the order is "processing". I need to change the default order-status to "pending".

How can I achieve this?

The default order status is set by the payment method or the payment gateway.

You could try to use this custom hooked function, but it will not work (as this hook is fired before payment methods and payment gateways):

add_action( 'woocommerce_checkout_order_processed', 'changing_order_status_before_payment', 10, 3 );
function changing_order_status_before_payment( $order_id, $posted_data, $order )
 $order->update_status( 'pending' );

Apparently each payment method (and payment gateways) are setting the order status (depending on the transaction response for payment gateways)…

For Cash on delivery payment method, this can be tweaked using a dedicated filter hook, see:
Change Cash on delivery default order status to "On Hold" instead of "Processing" in Woocommerce

Now instead you can update the order status using woocommerce_thankyou hook:

add_action( 'woocommerce_thankyou', 'woocommerce_thankyou_change_order_status', 10, 1 );
function woocommerce_thankyou_change_order_status( $order_id )
 if( ! $order_id ) return;

 $order = wc_get_order( $order_id );

 if( $order->get_status() == 'processing' )
 $order->update_status( 'pending' );

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

Tested and works

Note: The hook woocommerce_thankyou is fired each time the order received page is loaded and need to be used with care for that reason...

Now the function above will update the order status only the first time. If customer reload the page, the condition in the IF statement will not match anymore and nothing else will happen.

Related thread: WooCommerce: Auto complete paid Orders (depending on Payment methods)

// Rename order status 'Processing' to 'Order Completed' in admin main view - different hook, different value than the other places
add_filter( 'wc_order_statuses', 'wc_renaming_order_status' );
function wc_renaming_order_status( $order_statuses ) 
 foreach ( $order_statuses as $key => $status ) 
 if ( 'wc-processing' === $key ) 
 $order_statuses['wc-processing'] = _x( 'Order Completed', 'Order status', 'woocommerce' );
 
 return $order_statuses;