Document Javascript works in Acrobat, but not in Reader when called as a function from fields

I have a series of fields that are set to remain hidden until a particular calculating field's value goes above zero. I have a script at the document level that is called from the series of fields as a function in Custom calculation script (see below.) This works great in Acrobat but not in Reader. If I modify it where the script is directly in each field, it works in Reader. I use Adobe Acrobat 2020 and Adobe Acrobat DC Reader. I have attached the file.

function unhideFields() {

   var a = this.getField("txtAdjustmentAmt");
   var f = event.target;

   if (a.value > 0)  
   {
       f.display = display.visible;
   }
   else if (a.value <= 0)
   {
       f.display = display.hidden;
   }
}

Thank you,

Teagan