Confirming appointments not working

Greetings,

I recently downloaded your plugin "Appointments+". There are many wonderful features - however, recenlty, when I've tried to confirm appointments via the back end panel, it simply says "You don't have the authority to do this." Would someone be willing to tell me what this means? Strangely, I was able to confirm one appointment before it started to not work.

Thanks so much for your help!