https://www.pbainbridge.co.uk/2019/01/data-validation-check-for-blank-cells.html

1) Data Validation - check for blank cells.gs

function checkData(sheetName, sheet, lastRow, lastCol) {
  
  Logger.log('Last Row is: ' + lastRow);
  Logger.log('Last Column is: ' + lastCol);
  
  var data = sheet.getRange(2, 1, lastRow, lastCol).getValues();
  
  
  // loop through each Column ***********************************************
  for (var i=0; i<lastCol; i++) {
    
    // loop through each Row ************************************************
    for (var j=0; j<lastRow-1; j++) { // '-1' to ignore Column heading row
      
      // check for blank cells
      var errorFlag = false;
      if (data[j][i] == '') {
        Logger.log('Blank value found');
        
        // determine Column value to provide cell reference
        switch (i) {
          case 0:
            column = "A";
            break;
          case 1:
            column = "B";
            break;
          case 2:
            column = "C";
            break;
          case 3:
            column = "D";
            break;
        }
        
        Logger.log('Please check cell: ' + column + (j+2));
        
        // run validationPopup Function to display messge to user
        validationPopup(sheetName, column, j) 
        
        // stop script from continuing to run
        return errorFlag = true;
        
      }
      
    }// end of loop through each Row ***************************************
    
  }// end of loop through each Column **************************************
  
  // everything is ok and there are no blanks
  return errorFlag = false;
  
}