Installer for Sierra won't allow install on an external disk with no OS on it.

You cannot run the older macOS while booted to a newer version of macOS....Apple never considered the possibility of wanting to install to an external drive. Apple thinks you would only want to upgrade the OS from 10.11 or earlier. You would need to create a bootable macOS 10.12 Sierra USB installer, however, as @MrHoffman mentions that particular installer is broken when it comes to creating a bootable USB installer (it can be fixed by changing on character in a .plist file within the installer, but Apple can't be bothered to fix it.....Apple has been aware of the issue for many years now and actually hid the Sierra installer instead). Here is the fix for allowing a USB installer with the Sierra installer (see post by Eau Rouge):

Unable to make bootable USB drive - Apple Community

As @MrHoffmann mentioned, if your Mac can run 10.12.x, then it can run 10.13 High Sierra.

How to download and install macOS - Apple Support

Create a bootable installer for macOS - Apple Support

Try macOS 10.13 High Sierra. The macOS 10.12 Sierra installer has reportedly occasionally had some issues.

Download the installer, build a bootable installer, boot that, and use that bootable installer to install on the external.

Create a bootable installer for macOS - Apple Support

You Mac Pro can't run Sequoia. Sequoia requires a 2019 Mac Pro, a 2017 iMac pro, a 2019 Intel iMac or a Silicon Mac. The newest system your Mac Pro can run is macOS 10.14.6 (18G95) with recommended Metal-capable graphics cards.