

When the grey Apple logo appears, release the Shift key. Once you hear the startup tone hold down the Shift key on your keyboard. To do this, turn off your Mac and wait 10 seconds, the press the power button. 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. macOS Sierra problems: Not enough space to install

It might also be worth switching from a Wi-Fi connection to a wired connection before you download and install. Boot your Mac into Safe mode, then try installing macOS Sierra again. Press the power button on your machine while holding down the Shift key on your keyboard.
