The loop (Do Until...) reads values from the sheet row by row, then it looks for a subfolder of your default calendar with the name read from the sheet. And the error means that there's a name in the sheet for which there is no subfolder.
Place the cursor over the variable named "arrCal". A little label will come up telling you the current value of the variable. Let's say, the current value is "test". That would mean that your default calendar has no subfolder called "test".