how can i delete the hardisk after i format the mac and download new

Can you give us more details about that?

Are you trying to erase the Mac again, after you installed a new macOS?

In that case, use the Disk Utility from the Utilities window, before you continue to reinstall the macOS.

How to reinstall macOS from macOS Recovery - Apple Support

Or did you install a new internal hard drive and need to delete the old one?

How to erase a disk for Mac - Apple Support