OS X Base System only disk available

Hi! I had problems with my iMac and tried to erase the OS and re-install, but I did something wrong and now all I have is asingle partition with OS X Base System that is the size of my HD (1 TB), see picture 1. This new disk image cannot be partitioned: when I hit the + sign in the partition screen, nothing happens. In the Disk Utility, the options for erase, restore and unmount are all unavailable (see picture 2), I imagine that's because you normally cannot do any of these on the OS X Base System disk. This disk is also locked, so I cannot re-install High Sierra (see picture 3).

As the OS X Base System is the only thing left in the computer, every time I start it up, it does directly into the Recovery menu.

I've researched on the internet and a lot of people made this mistake. Most of them where able to either erase or re-install the system, but I'm at a dead end, as I don't have any of these options available. I don't know what else to try. Does anyone know how to fix this? Thanks!

Pic 1 - OS X Base System is the only disk available

Pic 2 - When I press the + sign nothing happens

Pic 3 - Cannot reinstall High Sierra as the only disk available is OS X Base System, that is locked