Disable Pop-Up Blocker
After the print file is generated and stored on your server, it will try to open the file in a new window tab. In most cases the pop-up blocker of your browser will block the window to open automatically, you will see a notification about that in the browser toolbar. Just disable the blocking for your website.
Conflict with other plugin
The print file is generated on our server and send through the WordPress REST API to your website. Some plugins are conflicting with our REST endpoint and blocking the request. Here are some plugins listed which are not working with our plugin. Please disable the plugin or contact the plugin developer how to stop the blocking.