How to create a sub branch?

In git, branches correspond to actual files in a hierarchy in the .git subdirectory. If you create a branch named bug/sub, git will first create a folder .git/refs/heads/bug (if it doesn't already exist) and within that folder it will create a file named sub which is the branch. Branch names are presumed to resolve within .git/refs/heads, so refs/heads + bug/sub resolves to a real place in the file system.

The trouble that you're encountering comes from your file system. Since you already have a branch called bug, in .git/refs/heads there is already a file named bug. Creating the sub branch as outlined above would require the file system to create a new folder named bug, but it can't because there is already a file there named bug.

In brief, you're free to create a hierarchy of branches, but higher-level nodes of your hierarchy can't be branches in their own right.

for creating a new sub branch
git checkout -b <sub-branch-name>
// sub branch will automatically created at the time of push
for pushing file on sub branch created earlier
git push -u origin <sub-branch-name> -u for setting upstream parameter

here origin is the master branch, added with

git add remote origin master https://github.com/<ur-name>/repoName.git

checkout the name of branches

git branch