BIGVU Teleprompter uses your phone's camera in order to record. There are a few reasons why this process might fail.
If you have an old phone, or a phone belonging to Oppo, Vivo or Realme, please make sure Battery Saver is off. You can find whether or not Battery Saver is on by going to your phone's settings, under "Battery" options.
A common issue among new android devices is the screen refresh rate. Newer Android devices having a 120 Hz refresh rate and for the app, you need to use a 60 Hz refresh rate.
See devices that using a high refresh rate here
In order to resolve this please do the following procedure :
From Device Settings, search for and select Motion smoothness.
Tap Motion smoothness again, and then choose Standard refresh rate - 60 Hz refresh rate.
for example (Samsung devices): https://www.samsung.com/au/support/mobile-devices/customise-motion-smoothness/
BIGVU will support 120 Hz soon.