Deprecated header warnings

I get the following header errors when activating the Purchase Orders plugin.

Deprecated: Required parameter $order follows optional parameter $sent_to_admin in /home/rossi/public_html/wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php on line 762

Warning: Cannot modify header information - headers already sent by (output started at /home/rossi/public_html/wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php:762) in /home/rossi/public_html/wp-includes/functions.php on line 6584

Warning: Cannot modify header information - headers already sent by (output started at /home/rossi/public_html/wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php:762) in /home/rossi/public_html/wp-admin/includes/misc.php on line 1310

Warning: Cannot modify header information - headers already sent by (output started at /home/rossi/public_html/wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php:762) in /home/rossi/public_html/wp-admin/admin-header.php on line 9

Any idea on how to solve these?