For other readers, there is a solution after all:
As you may already know, this plugin requires you to set a push notification icon which has to be put inside resources folder in the project root. However, it is impossible to add the file directly into that folder from Monaca Cloud IDE.
After a long searching around, I found a Cordova plugin, called cordova-plugin-copy-resources, which allows you to copy resource files at build time. I tested the plugin and successfully added the file under resources folder.
Therefore, in your case, please do as follows:
Add cordova-plugin-copy-resources to your Monaca project.
Add the following snippet to the config.xml file:
<!-- copy the www/resources/android_notification_icon.png into project/resouces/ folder during build time --> <resource dest="resources" src="www/resources/android_notification_icon.png" /> <!-- set the ParseNotificationIcon --> <preference name="ParseNotificationIcon" value="android_notification_icon" />
Save the config.xml file and start building again. This time the build should be successful.
So in the folder that is mentioned you should put your icon!