The problem here is that you’re using an unsupported install. We can only really support the standard install here. Third party packages break all the time due to changes occurring within Discourse itself - this is why they’re not recommended.
The version of the calendar plugin you’re trying to install isn’t compatible with the version of Discourse in the Cloudron package. Cloudron has repackaged Discourse and as a result aren’t using the supported means to install plugins. Unless you want to switch over to a supportable install they’re going to need to guide you in fixing this and update their documentation.