If you see the error 'no module named microbit' or similar when running Mu, you may need to switch the editor to Microbit mode. Plug in your micro:bit and follow this guide.

A mode button has been created as the first icon in the menu bar. Click on the Mu logo to open the mode selector dialogue where you can choose BBC micro:bit mode from the menu

Mu supports many devices running different versions of Python or MicroPython, the mode selector allows you to easily switch between these.