Unauthorized access error message

I tested the free version of this plugin on our site. When I try to send a test email and click the send button I get an Unauthorized Access error message. All of our other email works fine. Your help with this is appreciated- it’s a great plugin and the Pro version is exactly what we want- if it works.

https://www.ads-software.com/plugins/product-enquiry-for-woocommerce/