MPG Template cannot be edited or viewed - gives 404 error

If you're using a page builder and you're not able to edit the template of your project - it's expected. It happens because there is a setting that allows hiding a template from the website (so it doesn't appear on search results or anywhere in the loops of the website). You don't want to show the template with tags for the visitors, right?

If you're using this option, it's no longer possible to edit the template or visit it on the front-end so if it's needed to edit the template and change something, this option should be unchecked:
then you can edit the template and enable it again after finished. It's expected behavior because page builders allow editing the template in the front-end and it's no longer available in the front-end when this option is enabled.