As far as I know, there is no support for flashlight while we are chatting via video call in a smartphone. Why it is not present? Is there any hardware limitation? Can someone explain?
As of now, Android doesn't provide any option to enable flashlight during video calls.
Examples for iPhone and Blackberry are here.
The Mi forum question "How can i turn on front flash during video call - Redmi Y1 - Mi" says option #3 works. Another thread "[Q & A] How to start Selfy Light During Video Call????" claims it's a known problem and that you can fix it by resetting your phone and not enabling Face Unlock.
There's no reason the light shouldn't work and you can stand near a white wall or reflective surface to get enough light at night to make a video call. If you edit your question to include a specific phone model we may find a specific set of instructions for your phone.
If nothing works you can get around it by purchasing a "Phone USB Light":