Error message showing up on blog

I tried to load a new plugin and got this error message:
Notice: Undefined index: page in /home/content/54/5309754/html/pump-shoes/wp-content/plugins/order-categories/category-order.php on line 20

This is what I see when I open the admin panel:
Notice: Undefined index: page in /home/content/54/5309754/html/pump-shoes/wp-content/plugins/order-categories/category-order.php on line 20

Warning: Cannot modify header information – headers already sent by (output started at /home/content/54/5309754/html/pump-shoes/wp-content/plugins/order-categories/category-order.php:20) in /home/content/54/5309754/html/pump-shoes/wp-includes/functions.php on line 790

Warning: Cannot modify header information – headers already sent by (output started at /home/content/54/5309754/html/pump-shoes/wp-content/plugins/order-categories/category-order.php:20) in /home/content/54/5309754/html/pump-shoes/wp-includes/functions.php on line 791

Can anyone please let me know what’s going on here and how to fix it?

Thanks,

Cathy