Prøv at starte op i safe mode. Dette er ganske vist for Sierra, men burde stadig virke:
If you get an error message while installing macOS Sierra saying that you don't have enough hard drive space, then restart your Mac and boot into safe mode. To do this, turn off your Mac and wait 10 seconds, the press the power button.
Once you hear the startup tone hold down the Shift key on your keyboard. When the grey Apple logo appears, release the Shift key.
Once in safe mode delete any unnecessary files, or move them to an external hard drive, to free up some disk space. Then restart your Mac and try to install macOS Sierra again.