El Capitan download for Mac Pro - App store "system is too new" I have a Mac Pro 3,1, which came without a hard drive.
Now I want to install macOS El Capitan on a new SSD in this machine.
I started creating a USB-Stick with El Capitan on it.

I used this link: https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12 which leads me to the App Store.
But if I click on download, it says that it can't install macOS on my hard drive because the system is too new. 
The system I used to do this is my Mac mini 5,1 with Sierra.

How can I create, with this Mac mini, a USB Stick with El Capitan setup for my old Mac Pro?
You'll struggle... the 3,1 won't boot from USB.
People say you can but I've had one since new &amp; no amount of cajoling has ever made it do so. 

If you have El Capitan in your purchase history on the Mini, then that's where you will be able to download it. The version linked in the App Store can only be run on machines that cannot run Sierra.

If you have a clean external USB drive, you could get the installer onto it the same way as you would have onto a USB stick. If you then drop the drive in the 3,1 it will boot from that.
Alternatively, once you have the right installer, you can run it to the external from the Mini &amp; then swap it into the 3,1.