The sync process and the generation of the initial link code depend on the Bluetooth connection between your mobile and your watch.
Open the Fitbit app on your mobile and verify the sync details. You must see the text "a moment ago" near the watch icon. (take a look at the included picture)
The activation of the all-day sync option on the Fitbit app on your mobile phone can reduce issues.
A reboot of your mobile phone and watch usually solves Bluetooth problems.
You can find detailed connection issues troubleshooting at the Fitbit website.
Your Fitbit watch determines the temperature unit (Celsius or Fahrenheit) based on the preferred distance units (miles or kilometers) on your Fitbit profile.