Make sure Record Switch is turned On in the app, under SD Card Settings. This is located in the Settings menu on the upper right corner in the control panel.
Make sure that the microSD card is installed correctly. To confirm the camera is recognizing the microSD card, go to the SD Card Settings in the Settings menu and make sure microSD Card Capacity menu shows Total Capacity, Used and Free Space.
If this does not appear you may consider reformatting the microSD card.
WARNING: Reformatting microSD card will delete all videos stored on the card.