Hi @Chothiru, welcome to the community.
Habe you tried giving your Ionic a restart by following the steps here? It's also worth keeping the following in mind from the user manual.
Adjust your heart-rate setting
Track your heart rate day and night with Ionic. To preserve battery life, turn off heart-rate tracking in the Settings app > Heart Rate.
Note that the green LED on the back of Ionic continues to flash even if you turn off heart-rate tracking.
For more information, see help.fitbit.com.