You may not have python2.4 installed or not in $PATH .
virtualenv , other than rvm , does not install Python versions, uses only what you already installed. When you use the -p option, it means --python , you are telling virtualenv to create a new sandbox based on this Python installation.
So, if you have a Python 2.4 installation somewhere, just do:
$ virtualenv --python /path/to/python2.4/bin/python2.4 --no-site-packages ~/env/NEW_PROJECT
PS .: --no-site-packages deprecated in new versions of virtualenv . By default, it does not use global site packages.
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