Why can't I erase my MacBook Pro?

Should you have the original Leopard 10.5(.8) DVD media/installer

try starting up with Shift key, held and go through install/restore

instructions (find them first, if that mac is only one) online.

• How to Reinstall Mac OS X (Leopard 10.5.8 and Earlier): 15 Steps

https://www.wikihow.com/Reinstall-Mac-OS-X-(Leopard-and-Earlier)

Since you could install a new OS from a DVD, and not personalize

it, that would be OK; if donating erase Mac first. Otherwise, other

ideas may include re-installing to re-purpose older Mac.

Presently, I've 2-older Macs, (early build MacBook1.1 13-in; & last build

model G4 Mac Mini.) Both run Leopard 10.5.8, and use TenFourFox.

The reinstall can allow use of Disk Utility, to fully erase the Mac HDD.

System DVD may be ejected after, with or without another OS install.

Good luck & happy trails!🌻🐝

You can't erase the disk you are booted into.

What is it you are trying to accomplish? Maybe this: What to do before you sell, give away, or trade in your Mac - Apple Support