Hi Nishad,
I was having the same problem as the other 2 people.
I think the problem had to do with that I was originally using an older version of this plugin. After updating the plugin, to version 1.2.8 at the time of this writing, I was still having the same issue.
I fixed the problem by deleting the fixed rate. I then re-added the fixed rate back in, but this time I disabled it. Success. The Canada Post shipping plugin worked. I wasn’t sure if disable the rate was the fix so I tried once more and re-enabled the fixed rate, but Canada Post still works… Maybe all that needed to be done was deleting/re-adding it.
Hope this helps someone following this thread.