I realized that I should choose Standard runtime instead of Node.js even if the build target is a Node.js project because the default python version is 2.7.6 and pip is not installed on Node.js runtime by default.
I have checked with following buildspec.yml file.
version: 0.2 phases: install: commands: - python --version - python3 --version - pip --version - pip3 --version
With Node.js runtime:
The default python version is 2.7.6 and pip is not installed.
With Standard runtime:
The default python version is 3.7.2 and pip is installed.