Dear Mo,
I checked your input tags. You are setting ML_MODE=refit which is correct. But then you are also setting ML_ISTART = 2 which activates production mode and overwrites the ML_MODE tag. Therefore no ML_FFN file will be generated. In principle the ML_ISTART tag should not be used any more and will be replaced by ML_MODE tag.
Moreover the tag ML_FFNAME = ML_FFN does not exist in vasp. The generated ML_FF file will automatically be called ML_FFN by vasp.
All the best Jonathan