Instantaneous precip type requires two fields, RAINCV and RAINNCV, that are not output by default in the WRF model. You will need to modify the WRF Registry.EM_COMMON file to turn on the history for these fields and then rerun the model.
For the GSD precipitation type method, the required variables include PREC_ACC_C, PREC_ACC_NC, SNOW_ACC_NC, GRAUP_ACC_NC, SNOWNC, AND GRAUPELNC. You will need to turn on the history for these variables in the Registry.EM_COMMON before running the WRF model in order to have them in your WRF output.