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For testing, I would add
 Ouput:Variable,
  *,            !- Key Value
  Air System Fan Electric Energy;      !- Variable Name
 to the IDF, save the IDF after the change, and simulate one day to generate the output CSV file. In the CSV, check the names assigned to header columns for Air System Fan Electric Energy. You may have spelled something wrong in the Meter:Custom input fields for Key Name, or entered the name of the AirLoop:HVAC object when it is expecting the name of a different object (like the Fan on the supply side of the AirLoop:HVAC object).