Either manually write the version you need in your Pipfile:
[requires]
python_version = "3.6"
Or install it on your system. But I guess you will need the version to be installed if you plan to actually run pipenv install.
I would suggest to use pyenv: https://github.com/pyenv/pyenv.
Follow the installation instructions, then installing Python 3.6 is just a matter of
pyenv install 3.6.3
Then you can set the order of preference with
pyenv global system 3.6.3
Besides, if pyenv is available, pipenv will automatically use it to install the required version. From pipenv README:
Automatically install required Pythons, if pyenv is available.