Hello!
LTFM is a rather new airport. The problem you have when using the free version of Simbrief is that you only get access to an old AIRAC cycle. The AIRAC cycle is (easy described) the database with all the airports, waypoints etc.
As Simbrief only provides you with an old cycle, it doesn’t include the new LTFM yet. Therefore, you can’t create a FPL from or to this airport. Another example that doesn’t work is the new airport in China.
You would need a Navigraph subscription to always get access to the most recent AIRAC cycle.
All the best!