I am pretty new to Mac and I screwed up while updating my MacBook Air to macOS Sierra. When booting the Mac after the first restart in the installation, there was a 0-100 % installation bar and I accidentally pulled out the power plug to the Mac (the battery is not working). This resulted in what you call the "kernel panic". After some searching I have found the recovery mode/menu and tried to reinstall macOS, but I get the message that my Macintosh HD does not have enough free space. I then accessed the Disk Utility and tried to erase the Macintosh HD, but it failed. (maybe because I was in the middle of the installation wizard of OS Sierra?) I am now trying to install macOS Sierra on an USB drive, but I do not know exactly what to do when this is done (if it works)... Is it possible to make some free space on my Macintosh HD so that I can install macOS Sierra from the recovery mode? Is there anything else I can do to fix this problem?