Can't install osx (drive problems)

Hello, I'm just starting my adventure with a new friend (mac mini) and I'm very happy with it. Unfortunately, when installing windows on a different disk, something (probably when changing the partition (on which windows was supposed to appear) from gpt to mbr) went wrong and removed the entire content of the osx drive. When I reinstall my system, the target disk is not displayed. I am trying to restore the system by a disk tool (after booting with cmd + alt + r) but with first aid, partitioning, erasing ... whatever errors appear to me. The disk shows the FAT file system (which I can not change) and shows up in gray.