DB Design :: Changing Database In Memory

I need to change the table codes to in memory optimized but every time I try to change I got an error message like you need to change database to memory optimize.

So, I try to change it  using this code:

ALTER DATABASE Coralreef_ ADD FILEGROUP Coralreef__mod CONTAINS MEMORY_ OPTIMIZED_ DATA.

When I used this  I got an another error:

Msg 10797, Level 15, State 2, Line 1..Only one MEMORY_OPTIMIZED_DATA filegroup is allowed per database.

So, How can I change database to in memory optimized database.