Hi,
For all who have the same problem as me, i just find a way to connect Ikea Tradfri with Flic. First, make sure that you have all your apps updated. I've uninstalled Flic app and reinstall it. And now Flic app accept my Tradfri accessories.
Hope this help 
Jonathan
Update : I've tried several times to switch off/on or reset, it's the same problem. I will wait for your investigation
Thank you
Jonathan
Hello
Thank you for the response. I will try your method.
Jon
Hi everyone,
I’m coming here before I will pluck my hair 
I have the first flic button and I’m trying to connect it with the IKEA Tradfri gateway. The flic App found my IKEA gateway but when I have to enter the security code, it says that it’s wrong.
When I installed my gateway, I had to enter the security code and it works so I’m sure it’s the good one.
Did you ever seen this kind of problem? I’m going crazy.
Thanks 