Nan

[Leesam]

There is no display on the face of Fitbit except date and time. Heart rate not showing neither steps etc. but I can see on the phone 

[JohnnyRow]

That usually means there is a problem with your clock face.  Try switching to another clock face, a Fitbit one is safest.  'Nan' is computer error for 'not a number'.  Non-fitbit clock faces sometimes develop errors, perhaps from developers not keeping up with fitbit changes.

If a fitbit clock face works, then you can experiment with other choices, knowing you can go back to a fitbit one that will work.