If you got your pass in September 2021, I assume you got it based on completed vaccination doses without a booster. If that's the case, it's likely that the certificate has expired because too much time has passed since your initial set of vaccinations (when were these completed?) and the certificate does not reflect any booster you may have gotten since.
Unfortunately if that's the case, there's no way to update the certificate without getting a new one.
However! This is not a big deal or a problem. For purposes of entry to France, as long as your initial set of vaccinations was either within the last 270 days or if you have received a booster, then you are good to go. As long as you have your Canadian proof of vaccination, you don't need a EU DCC to enter France (particularly since France currently only issues DCCs upon application in person at a pharmacy in France anyway!) The only minor issue is that you may not be able to use Verifly, in which case you will have to get your documents checked manually at the airport.
Furthermore, once you arrive in France it also won't be an issue, because the use of the vaccine pass to enter museums, restaurants, etc. will be suspended effective March 14. So by the time you go in early April there will be no need to show the pass once in France. (There are some residual requirements for a health pass to enter medical centers or nursing homes but I assume these are not on your itinerary!)
