Regarding

var numberOfDays = testDates('BH2','BL2');
Logger.log(numberOfDays);
function testDates(DateLow, DateHigh) {
  var now = playgroundSheet.getRange(DateLow).getValue();;
  var nowMilli = Number(now.getTime()).toFixed(0);
  var targetDate = playgroundSheet.getRange(DateHigh).getValue();
  var targetDateMilli = Number(targetDate.getTime()).toFixed(0);
  var durationMilli = targetDateMilli - nowMilli;
  return numberOfDays;
}

replace

var numberOfDays = testDates('BH2','BL2');
Logger.log(numberOfDays);

by

function myFunction(){
  var numberOfDays = testDates('BH2','BL2');
  Logger.log(numberOfDays);
}

Then run myFunction instead of testDates.

The above because calling testDates or any other function from the script editor, a custom menu, etc. will cause that the statments in the global scope be executed before running the function that was called, in this case, it ran two times, the first is caused by var numberOfDays = testDates('BH2','BL2'); the second by the UI/trigger used to call the function. The error occurs because the call from the UI doesn't pass parameters to the function.

Try it this way:

function testDates(DateLow, DateHigh) {
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getSheetByName('pg');
  var now = new Date(sh.getRange('BH2').getValue());
  var nowMilli = now.valueOf();
  var targetDate = new Date(sh.getRange('BL2').getValue());
  var targetDateMilli = targetDate.valueOf()
  var durationMilli = targetDateMilli - nowMilli;      
}

Thanks to TheMaster and Rubén. The final function called from another function returns the number of days between two dates:

function testDates(DateLow, DateHigh) {
  var now = playgroundSheet.getRange('BH2').getValue();;
  var nowMilli = Number(now.getTime()).toFixed(0);
  var targetDate = playgroundSheet.getRange('BL2').getValue();
  var targetDateMilli = Number(targetDate.getTime()).toFixed(0);
  var durationMilli = (targetDateMilli - nowMilli).toFixed(0);
  return durationMilli/day;
}

I have been reading here a lot as I try to learn the language and this is my first question. Absolutely amazed at the quick and helpful responses. Forever learning, David