pip: Could not find an activated virtualenv (required)

@Bibhas has it; +1 to look for export PIP_REQUIRE_VIRTUALENV=true in ~/.profile or ~/.bashrc. You can confirm the setting in your current shell with env |grep PIP_REQUIRE_VIRTUALENV.

This setting is a good safety check; more often than not, you'll want to be installing things into virtualenvs. However, sometimes you do want to be working with the global/system python. In those cases, take a look at --isolated:

Run pip in an isolated mode, ignoring environment variables and user configuration.

$ pip install --upgrade pip
Could not find an activated virtualenv (required).
$ pip install --upgrade pip --isolated
Requirement already up-to-date: pip in /usr/local/lib/python2.7/site-packages
$ pip freeze --isolated
...

Open your ~/.bashrc file and see if this line is there -

export PIP_REQUIRE_VIRTUALENV=true

It might be causing the trouble. If it's there, change it to false and run -

source ~/.bashrc

If not, run export PIP_REQUIRE_VIRTUALENV=false from terminal.

Note: everything works the same if you have .bash_profile instead of .bashrc in your current user's root directory.