Android Studio が GIT をコミットするとき、このリポジトリでは別の git プロセスが実行されているようです（例：エディタを開いている）。

Android StudioがGITをコミットすると、次のような例外が報告されます。

File exists. Another git process seems to be running in this repository, e.g. an editor opened by 'git commit'. Please make sure all processes are terminated then try again. If it still fails, a git process may have crashed in this repository earlier: remove the file manually to continue.

解決策

Execute in the root directory of the project, which is also the directory where .git is located (Android Studio Terminal execution)
rm -f .git/index.lock