I'm sorry you're going to miss being paid for the shop. You sound like a new and relatively inexperienced shopper. I'm sure you worked hard on the report. But as an independent contractor, we accept jobs with guidelines. In order to be paid for a shop, the guidelines must be followed. I am assuming that you read the shop instructions before you made the call and you knew that the shop required that you make an appointment, that the shop must be done on Saturday, and the shop must be done with the representative you spoke with. You said when you made the appointment, "she told me that she would not be there on Saturday, so the appointment was scheduled with another representative." So you knew when you made the appointment that you would not be able to complete the assignment as required. At that point, I would have called the scheduler immediately and notified him I could not complete the shop according to the guidelines. The scheduler would have either told you to go ahead, that it would not be a big deal OR would have told you to cancel the appointment and reschedule.
I have done shops for NWLPC and the mistake you made is a big one for that shop. Yes, some MSCs will accept minor errors. Some subtract from your pay for errors. Others make a determination about how big the error is. In other cases, if the error is something that is a client requirement, the MSC is unwilling to submit a wrong report to a client because it makes them look as though they can't do the job. It all depends on how bad the error is, what type of error it is, and both the MSC and the client.
I'm sorry you had difficulty, but there is a learning curve to mystery shopping. My advice would be to accept that the shop was not performed according to the guidelines and learn from the mistake. I would say to read the guidelines for all shops and if something comes up and you know in advance that you are unable to perform the shop according to the guidelines, DO NOT DO THE SHOP. Contact your scheduler immediately.