The error message means that RapidMiner cannot resolve the repository location used inside your Retrieve
operator. The reason is that you have loaded the process from a file rather than from a repository. In fact, the process is stored in a repository, but the way you have loaded it, RapidMiner cannot know this. If you use this code to lode the process, it will be associated with a repository, and relative locatins can be resolved:
RepositoryLocation loc = new RepositoryLocation("//Repo/path/process");
Process process = new RepositoryProcessLocation(loc).load(null);