AMC doesn't appear to have made them available on demand to Canadians yet. AMC is one of many US channels where the extreme majority of their shows are not available on demand outside of the US. AMC does have streaming agreements for the first 10 seasons of TWD with Netflix in Canada so it's hard to say for sure which platform season 11 is going to appear on first.
The reason you can see Episode 12 as available is because it has live air dates in the future (tomorrow, Sunday and Thursday the 24th). I will check with our On Demand team to confirm, but my suspicion is that AMC (the content rights owner) has restricted access to On Demand content since the 2nd half of season 11 began. They launched their own streaming service AMC+ in Canada in summer 2021, and the number of titles made available On Demand has shrunk considerably. They have also sold the rights of back seasons to Netflix, so that's why seasons 1-10 aren't visible either.