As the error message says, you can't redirect a domain to itself.
I think you are trying to set a sub-directory as your document root directory. You can try to set DH panel --> Domains --> Manage Domains --> Web Hosting --> Edit --> Specify your web directory
By default, it is /home/username/zrpg.net. You can change it to /home/username/zrpg.net/forum
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