Weird. I don't remember if I got the same thing or maybe I ignored it as when I sent my application, I did fill out IMM 5476, stating myself (sponsor) as a representative. I am wondering if you filled out that form as well, then it will make sense. Otherwise, I don't know what to tell you other...
Try with Application#, if it doesn't work, try by first name, last name, passport#, DOB. In the last question, Enter 2 (Sponsor + PA). If it still doesn't work, try the new tracker tool. https://prt-srp.apps.cic.gc.ca/en/register
I believe this is the order:
1. AOR (Acknowledgement of Receipt stating that the office has received your application)
2. BIL (Biometrics Instructions Letter)
3. MR (Medical Request)
4. SA (Sponsorship Approval) meaning the sponsor is eligible to sponsor you...