Hi @Ishanth263430
Welcome to the forums.
With opencv there is this finiky step where you need to rememeber to make a build folder.
cd ~/opencv_build/opencv
mkdir build && cd build
That’s where PATH finds CMakeLists.txt.
I always forget this step.
Every single time I try to install openCV I have to google it and I always find this exact same stackoverflow thread. Every… single… time… 
Even answering this question, I had to google it to remind myself of the solution.
Pix 
