Erasing Disk - Could not open device

It sounds like you are trying to erase it either booted up into it normally, or booted up into regular Recovery Mode (Utilities) off of the hard drive itself. You aren't able to erase it in either case since you are booted up off the drive you are literally trying to erase.

To wipe it out and reinstall it fresh for your Aunt you'll want to boot up into Internet Recovery Mode (OPTION-COMMAND-R or SHIFT-OPTION-COMMAND-R, if it was running or a more recent macOS).

Either way, it takes about 5-10 minutes to load via Internet-based recovery, but that gives you the full unfettered access to the drive this process requires. You should see a spinning globe similar to this:

If it comes up in only a minute or less, you are not in Internet Recovery, but in normal recovery and should try the other keyboard combination.

You can follow these directions:

How to reinstall macOS from macOS Recovery - Apple Support

Make sure you complete these steps before gifting your MscBook Pro:

What to do before you sell, give away, or trade in your Mac - Apple Support

Hi! Have you tried booting via internet recovery mode (can do so by booting and hold down theCommand (⌘) – Option (⌥) – R combination) and then wiping it via disk utility?