Reload OS X EL Capitan - "item is temporarily unavailable"

[pdsingle]

I was given an old MAC Pro desktop computer. I had to erase the drive before leaving with the computer. Now it won't let me reinstall the OS. It is trying to install El Capitan. It is saying that "item is temporarily unavailable".

I did not have an Apple ID prior to getting this Mac Pro.

How do I get the OS to install back on the drive?
I have no access to another MAC.

 

[Synchro3]

You cannot download macOS from the internet with the cMac Pro.

In the assumption you have a Mac Pro 4,1 or 5,1:

The official way: Buy the Snow Leopard DVD or the Lion USB stick from Apple:

https://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard?
or
https://www.apple.com/shop/product/D6106Z/A/os-x-lion?

Start the Installer from the DVD or USB drive and install macOS on the internal drive. Format the drive HFS+ (journaled) if necessary before.

Once installed, open an Apple ID Account: https://appleid.apple.com/#!&page=signin

As older macOS are no more indicated in the App store; direct Links for macOS

El Capitan: https://itunes.apple.com/us/app/os-x-el-capitan/id1147835434?mt=12

Sierra: https://itunes.apple.com/us/app/macos-sierra/id1127487414?mt=12

High Sierra: https://itunes.apple.com/ch/app/macos-high-sierra/id1246284741?mt=12

Install the macOS of your choice.

The newest macOS Mojave can be downloaded normally from the App store. However you should consider that Mojave needs a Metal compatible graphics card, like Sapphire RX 580 Pulse 8GB: https://support.apple.com/en-us/HT208898