Hi there!
This is happening because the game is not finding the character named $gen_test_gen.png. Please, press F8/F12 when the error happens. Go to the console TAB and send me a screenshot of the error log there.
1 - Since this is a complex plugin, please first try it on the Sample project and make it work there. After you manage this, implement it in your project and see if it works.
2 - If you already did that, you will need to show me the steps you made to get there: Did you create added new piece files to create another generator? Did you use the generator that already comes with the sample project? Show me a screenshot of your plugin command, that you used to create the character and the one that you used to apply the Generated character into another one.
3 - Also, if you added the plugin on a current save that previously did not have the plugin it will not work. You need to start a new game when you first added this plugin to your project.
4 - I can also see that you are using other plugins, maybe this 3D plugin can also be a compatibility problem. If everything did not work, you can try disabling it to see if they have any compatibility problem.
If none of the above works, added me on discord: hakuenstudio and send me a project replicating the issue so i can take a look at the problem.