I am trying to sell and old MAC late 2012. Have had to go the macOS Recovery and erased the disk and then when to Reinstall OS X. it wants to install OS X El Capitan that is no longer available to download. I have then backed up my MAC Pro on Time Machine
The official word from Apple is if you are preparing a Mac for resale to install the OS it had when it was new.
What to do before you sell, give away, or trade in your Mac - Apple Support
If however you want to install Sierra it is still available, How to upgrade to macOS Sierra - Apple Support
if your Mac can run Sierra it can run High Sierra, How to upgrade to macOS High Sierra - Apple Support
Remember that any OS you install will be tied to your Apple ID and may be problematic for the new owner.
You can make a bootable USB installer which I believe will remove the association with your Apple ID.
Follow Apples instructions here, How to create a bootable installer for macOS - Apple Support
or you can use DiskmakerX which does all the work for you, http://diskmakerx.com/whats-this/
Make sure you follow the instructions for the OS you want to install.
And El Capitan is still available to download, How to upgrade to OS X El Capitan - Apple Support
Do you really have a Mac Pro (2006-2012, Silver Towers, 2013, Trashcan) or is it a MacBook Pro
a lot of people make a mistake with that on here.
If it is a Mac Pro Silver Tower then it is possible that it came with original grey install disks,
do you have them.
The official word from Apple is if you are preparing a Mac for resale to install the OS it had when it was new.
What to do before you sell, give away, or trade in your Mac - Apple Support
If however you want to install Sierra it is still available, How to upgrade to macOS Sierra - Apple Support
if your Mac can run Sierra it can run High Sierra, How to upgrade to macOS High Sierra - Apple Support
Remember that any OS you install will be tied to your Apple ID and may be problematic for the new owner.
You can make a bootable USB installer which I believe will remove the association with your Apple ID.
Follow Apples instructions here, How to create a bootable installer for macOS - Apple Support
or you can use DiskmakerX which does all the work for you, http://diskmakerx.com/whats-this/
Make sure you follow the instructions for the OS you want to install.
And El Capitan is still available to download, How to upgrade to OS X El Capitan - Apple Support
Do you really have a Mac Pro (2006-2012, Silver Towers, 2013, Trashcan) or is it a MacBook Pro
a lot of people make a mistake with that on here.
If it is a Mac Pro Silver Tower then it is possible that it came with original grey install disks,
do you have them.
Try booting into Internet Recovery (shift - command - option/alt - R on a restart). It is a slow process. This will install the macOS that came with your Mac, or the closest version still available. Reformat the drive using Disk Utility/Erase Mac OS Extended (Journaled), then click the Option button and select GUID. Quit Disk Utility. Then install the OS.
Cant install OS on old 2012 MAC after wiping hard drive
