what is ASP code for FP

I have just received my I-485 receipt notice from VSC. Following the instruction on the receipt, I tried to make an appointment for Finger Print. But the officer asked for a “ASP code”, she said that it is a code of one letter or one number on a column the receipt. There is not such code in my I-485 receipt at all. Anybody know what is this? How to schedule the appointment without such a code? Thanks for your information.