page not found error upon button click

HI,
I just installed and setup this plugin. When I click the buy button, I’m getting a page not found error for this link
http://sierracommons.org/wp-content/plugins/seamless-donations/seamless-donations-payment.php
It looks like that file is in the correct place.

Any ideas? I’d love to use the plugin.

Here’s the page where the shortcode is entered.
http://sierracommons.org/support-us

Thank you,
Evelyn

https://ww.wp.xz.cn/plugins/seamless-donations/