Do you have a second mac system which you can connect to this system via FireWire? [https://computers.tutsplus.com/tutorials/understanding-the-applications-for-target-disk-mode--mac-60609|Understanding the Applications for Target Disk Mode]
What you want to do here is reformat the drive to GUID journaled file system and then setup the proper OS onto your system. OS-X El Capitan 10.11.6 is the highest this system can support.
Get the needed OS installer from here [https://support.apple.com/en-us/HT211683|How to get old versions of macOS] and follow this guide to setup a bootable OS installer if your system is running something newer on your second system [https://www.macworld.com/article/2981585/how-to-make-a-bootable-os-x-10-11-el-capitan-installer-drive.html|How to make a bootable OS X 10.11 El Capitan installer drive]