Air France false advertising scam, lack of attention to details, or
"we just don't care that you spent thousands with us to get a reward
ticket"? You decide.
It's none of the above. It's just an example of one of the countless obscure little rules that are part of the larger game of loyalty programs/frequent flyer programs/award seats. It's a game that strongly favors the house (like any casino) and often frustrates those who play and end up with less than they had hoped for.
I'm sorry that you had expected to do this and only found out late in the process that it's not possible, and I can understand your frustration. But many (all?) airline frequent flyer programs tend to have complex rules, each has their own quirks (which sometimes work in your favor, sometimes they work against you).
You are not allowed to purchase that extra seat with the 25% discount
we advertise...
I bet if you look at whatever advertisement you are referring to, you'll see an asterisk or some fine print from the lawyers noting terms, conditions, exclusions. If not, ask for a full refund of your points and all fees, and start over with Plan B.
When you are using an award ticket, and are looking for something that's even slightly non-standard, you really can't just assume that things will go the way you would hope or expect, unless you ask specifically and you receive a reliable answer, or you do solid research to verify the details you care about - in advance. The "extra seat" option that you speak of is not a common thing, and to be honest, although I only have a little experience with Air France's program, I would not necessarily expect the "buy an extra seat" option - at a discount - to work with an award ticket. Lots of things don't work with award tickets, they are often treated as a "special case." They can be great, they can be a disappointment, often they're a mix or somewhere in between.
As suggested above, your best bet is to try and make the best of it that you can, but your options will probably be very limited and constrained. IME Air France is not a terribly flexible company, and employees may not always be empowered to "just fix the problem" for customers - I've seen other airlines do better at that (Air France still owes me a refund for a cancelled business class flight home from Istanbul when the pandemic blew up and flights were being cancelled by the hundreds...I don't expect them to ever come though with that refund and I've given up hope they will).