vielleicht hilft das hier weiter:
You need to boot into Internet Recovery Mode (Command + Option + R) or by booting from a bootable macOS USB installer in order to erase the drive. You cannot erase the same drive you are booted from so you need to be booted from the OS installer.
If your laptop can still boot, then I highly recommend creating a bootable macOS USB installer so you have more options in case something goes wrong with Internet Recovery Mode. Here is an Apple article for creating a bootable macOS USB installer:
How to create a bootable installer for macOS - Apple Support
Here is another article on how to reinstall macOS (it references Recovery Mode, but the instructions are the same once you are booted to the macOS installer):
How to reinstall macOS - Apple Support