So you got a link connecting you to an online available SampleApp (not just a OAC instance).

Then it's like said before: a RPD or a session variable can only be created in the RPD. In the front-end only presentation variables can be created. You can try to connect with Admintool to the host they give you, but if they didn't provide the port number and right credential it will just not work ...

You can reply them asking how you can reproduce an issue with a repository variable if they don't give you access to the RPD to create a repository variable first. (Or maybe they expect you to use a presentation variable instead, but sounds a bit dumb to me because it doesn't mean you will get the same behaviour)