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Are you wanting to save the volume average value or the like? If so have a look at monitors (and reports). 

The first part of the command makes sense, /file/write-data myfile.dat  should give you a Fluent data file. I don't recognise %d, but use %i, %t & occasionally %f   The  glass-granular phase-volume-fraction yes "Volume_Fraction_Step_%d.dat" part looks like ChatGPT has stuck two or three different options together to make up a result: the parts are correct in their own menu levels but not in the way it's written.