why I can't use void main()....when I am doing program in this app

Answer 1

because main needs to return a value to the operating system

Answer 2

This question is asked a lot of time here on this forum and has been answered a lot of times too.

By the way, on the website of the author of c++ it's said in the FAQ that void main is not c++. So the best is to avoid it, even if some compilers allow that, it is not standard...
A program should return 0 when everything has been done with success and an other int value to refer an error code when something got wrong.

source : http://www.stroustrup.com/bs_faq2.html#void-main