What you ran into is award seat availability. It looks like Air Canada had not made any award seats available on flights that would be more convenient... guessing those are flights during the business day that may have a tendency to be sold.

Oftentimes, the day of your flight, you can request to go on an earlier standby. I am not familiar with Air Canada specifically, but would expect they would consider a request if space is available. You may want to check out the Air Canada forum.... so great advice can be found there.

One caveat... to help your chances.... try to only have carry on bags... no checked bags. It is more difficult for an airline to accomodate your request at the last second if your bags are on an earlier flight. Of course, with so many flights before your scheduled one, checked bags may already be on an earlier flight.

Good luck!