When you get a message similar to this one in your browser console:

fieldtype-wrapper.js [Vue warn]: Failed to mount component: template or render function not defined.

found in

 ---> <CustomPlugin>

The reason can be that you most probably forgot to set the name in the code to the same you used in the plugin editor for your plugin.

You need to check here in the plugin editor what you used as the name and enter the same name as the value for the plugin property:

This can also happen when working with the https://github.com/storyblok/storyblok-integration-field-starter and deploying it with the wrong plugin name used in it.