I really don't want to come across as flippant, but are you really expecting Air France to fix their well-documented IT issues because you are mad that you couldn't use one specific debit card?

I have the same problem with RATP when trying to purchase carnets with a specific credit card through the app. Know what the fix was? Use a different credit card and save yourself the aggravation of typing out letters and emails that won't do anything.

If you insist on sending them messages, I would tighten up the above message along the lines of "Im having trouble using card X, my bank states that it is due to issues with your Verify process being non-compliant blah blahy blah. Please forward this to your IT department etc etc". Leave out the threats of changing airlines, it will fall on deaf ears.