Probable you use the old Flatcam version (cloning the Master branch). At some point JP Caram took the extension out so you need to do: `python FlatCAM` so no extension. BTW, for the version in the Master branch you need now Python 3.x and PyQt4. Afaik PyQ4 package is no longer available for Linux/MacOS so you need at the least to switch to the `old_flatcam_pyqt5` branch which is a branch based on the PyQt5 package.