First get the OS. You can get it here: http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg

After download, Install it on your Macbook(Another Mac with working OSx installed) to prepare the bootable USB.
If you encounter any package verification issues try this command in terminal:

Next, type the follow command to make a bootable USB:

Replace Untitled2 with the drive name for your USB.

After the drive is ready, Insert it to the Mac you wish to format and boot.

Helpful links:
https://www.appledollars.com/create-osx-usb
https://apple.stackexchange.com/questions/216730/this-copy-of-the-install-os-x-el-capitan-application-cant-be-verified-it-may-h

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