Thank you for your reply.
I did try to look deeply in the online apple support page, before contacting them via chat, in the chat they told me how to enter into recover mode: with my macbook air doesn't work the Command+R, I have to keep holding the power button (which is the finger print reader) when I switch the laptop on. So I did, I entered in recovery mode, I checked that the security settings were not modified. Still when I try to add a card I receive the same message "Apple Pay has been disabled because the security settings of this Mac were modified." I will try to contact Apple Support again...