As the error message states, the 'Specify Indices' mode of the MultiPort Switch block is not supported for HDL code generation. To implement this behavior you will need to decompose the logic you want to use blocks in their supported modes. If you have a small number of contiguous ranges this task is not overly difficult. If you have a large number of potential outputs, or the values to select a particular index are non-contiguous, the task becomes more challenging.
For example: if you want to select signal A when your select signal is 1-4, B when it's 5-8, and C if it's > 8, then you can implement this with 2 Switch blocks, 3 Compare to Constant blocks, and a handful of Boolean operators. If your selection logic is more complex, then the required design is more complex.
