Hey vinod, I have successfully downloaded android os source code for master branch.
See this link and do all procedure described in it :
https://source.android.com/source/building.html[
^]
The size of master branch source code is 31.5 GB
I think in your case the problem is, your source code not downloaded properly.
Because after successfully downloading the source code, all source files and folders must be present in your working directory.