@dorly are you trying in your self serve or by calling 611?
Try another set of 12 digits on the voucher to see if that works by either method. If not, wait a couple hours to try again.
If issues persist after a waiting period (sometimes 24 hours), submit a ticket to CSA for assistance.
To contact the Public Mobile Customer Support Agent (CSA)_Team, there are two methods to reach them found here:
It depends on where you purchased your vouchers as to their validity. Read the following linked post and reference the voucher catalogue link contained within for examples of vouchers. Be sure to use the 12 digit voucher PIN# as there are often more than one 12 digit # on vouchers.