reasons: workon command fails, the Python version changes, is to use the previous python3.6 install Python virtual environment, relevant documents is in the/Library/Frameworks/Python framework Versions/3.6/bin
solution 1: to install a virtual environment in new python3.7 environment

$sudo pip3 install virtualenv virtualenvwrapper
$source .bashrc

the solution 2: will/Library/Frameworks/Python framework Versions/3.6/bin three files in the directory, copy it to/usr/local/bin directory in

$cd /Library/Frameworks/Python.framework/Versions/3.6/bin
$cp virtualenv-clone virtualenvwrapper.sh virtualenvwrapper_lazy.sh /usr/local/bin