The line items in WooCommerce orders are only editable when the order is in a “Pending” or “On Hold” status. In general this makes sense. If an order is processing, it means the payment has already been accepted and any line item changes would likely require a refund (using the refund functionality) or a new payment (likely a separate order).

However, some business may have products that are frequently swapped or changed while an order is in processing, and it doesn’t make sense to have to switch the status to update the order. In those cases, you can use the wc_order_is_editable filter.

function devpress_processing_orders_editable( $is_editable, $order ) {
    if ( $order->get_status() === 'processing' ) {
        $is_editable = true;
    }

    return $is_editable;
}
add_filter( 'wc_order_is_editable', 'devpress_processing_orders_editable', 10, 2 );