Try this then.
https://eservices.cic.gc.ca/epay/
This should work.
Enter the appropriate details and you'll see a page that asks you to login. You need to click on "Register" and create an new account with your email ID. After that, you can pay the RPRF.
You'll get a receipt after your pay RPRF. I think you need to upload that in the appropriate placeholder in your myCIC account.
Also, as a bonus, let me tell you that since you have received your RPRF request, your application is almost through
. CIC usually does NOT issue RPRF if they are planning to reject you somewhere down the lane.