Hi @Ecat This happens when you try to enter the code two times in a short period, Can you please try to wait 2-4 hours before try again. If this doesn't help please contact the support, it is easy go on https://fizz.ca/en/support and click on the green bubble on the bottom right.
If you don't have a referral code, you can always pick mine, you can find on my name, the last 5 characters or you can get one from here https://forum.fizz.ca/en/discussion/2198337/this-is-the-place-for-your-referral-codes-here-and-only-here
If your SIM card not working, just contact the customer support and they will send you another SIM card in 1-2 days
As @elena code xzi4t said, it could be due to several attempts made in a short time... try to wait a bit and retry otherwise better contact customer support to assist you with that