The 'Structure' button appears to be greyed out initially after the data is loaded into the Neuro-Fuzzy Designer, indicating that this option is currently unavailable. The reason for this is that the initial FIS has not yet been created, and thus there is no ANFIS model structure to view.
However, once the desired number of membership functions and the types of membership functions for the input and output are selected, the initial FIS will be generated, and the 'Structure' button will become active. The subsequent steps involve training the FIS and testing its predictions against the loaded data.