Which version of ELKI are you using?
This error message usually indicates an issue with the distance parser used by the matrix reader.
Since ELKI supports different valued distance functions, DBSCAN cannot parse the epsilon parameter until the actual distance value type is known (which will provide the value parsing function).
Any earlier error message? If you have any earlier error (including missing required parameters), it will prevent DBSCAN from being able to parse the value.
Try setting the epsilon value last, and also try the command line. In the MiniGUI, due to the incremental way the parameters are set, these dynamic-typed parameters unfortunately can be flaky. Any patch to improve the handling of such parameters is appreciated.