I do not need to clear blank cells or anything. Just need to delete data from Row 800 to the Last Row.

My current code is as below, but it keeps return Range Not Found. How do I do it?

  function DeleteOld() {
    var spreadsheet = SpreadsheetApp.getActive();
    var lastRow = spreadsheet.getLastRow();
    spreadsheet.getRange('800:lastRow').activate();
    spreadsheet.getActiveSheet().deleteRows(spreadsheet.getActiveRange().getRow(), 
    spreadsheet.getActiveRange().getNumRows());
  Logger.log('Data Before 1990 deleted') 
  };

I tested the getLastRow function to see its number.

var spreadsheet = SpreadsheetApp.getActive();
Logger.log(spreadsheet.getLastRow())   };

The return is "12344.0" If I changed the code to getRange('800:12344"), it works well. If getRange('800:12344.0'), returns error.

Since the return of the getLastRow hass decimal point, could it be the reason? and How do I fix that?