The reason is...
In the beginning, I tried to edit the txt-survey since I need to do a looping in my survey , I copied question groups manually but I find it's really time-consuming to copy and import and change the question codes one by one, then I found this solution in the forum. But when I did it the error comes, I'm not sure if its because of my wrong operation, therefore I follow the manual to delete the "Mandatory column" to see if it would work:
"Testing mandatory questions. A common complaint is not the need to make many questions mandatory, but the need to turn off the mandatory feature for testing. Simply create the master spreadsheet with mandatory set to the final desired values. Then, to test it, just delete the "mandatory" column and save the test version of the spreadsheet. When you import that version, none of the questions will be mandatory. After you have finished your testing, import the master copy."
Yes but if the feature can really work it will really save time.