After replacing the battery in the Peugeot 206 key, 2004. it wont start

I have recently had a Peugeot 206 from 2004, the remote control for opening and closing the door did not work (one button was damaged and the other was not at all). I bought a new housing and battery, after replacing the door closes and opens without any problems, but the car does not start, it worked earlier. A spare without a remote control fires without any problems. What can be done to fix it without having to carry 2 keys?
Thank you in advance for your help.

There is an immobilizer transponder in the old housing. It looks like a small plastic wedge.
If you haven't already thrown it, put it in a new case. Otherwise, you will have to buy a new one, plus programming with someone who deals with French cars. Some car key makers have this stuff.

Thanks for the answer, you had a flare, I didn't change it earlier, now I did it, but in the new housing it is a bit loose. In the morning I will check whether it works, despite the fact that it is a bit loose.