Installing OS High Sierra on external SSD? Really need help with this

I don't understand why APFS isn't an option.

By Recovery I mean...

1. Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

2. Decide whether to erase (format) your disk

If you need to erase your disk before installing macOS, select Disk Utility from the Utilities window, then click Continue. You probably don't need to erase, unless you're selling or giving away your Mac or have an issue that requires you to erase. Learn more about when and how to erase.

3. Install macOS

After starting up from macOS Recovery, follow these steps to install macOS:

1. Choose Reinstall macOS (or Reinstall OS X) from the Utilities window.
2. Click Continue, then follow the onscreen instructions to choose your disk and begin installation.
3. If the installer asks to unlock your disk, enter the password you use to log in to your Mac. If it doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk. 

1. Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

If your Mac restarts to a setup assistant, but you're selling or giving it away, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

If you never installed macOS Sierra 10.12.4

If macOS Sierra 10.12.4 or later was never installed on your Mac, macOS Recovery works differently:

• Command-R is still the recommended way to start up from macOS Recovery. This combination makes sure that the installation isn't associated with your Apple ID, which is important if you're selling or giving away your Mac.
• Option-Command-R installs the macOS that came with your Mac, or the closest version still available.
• Shift-Option-Command-R isn't available.

How to reinstall macOS - Apple Support 

So, you're not booting from the USB stick???

in macOS Recovery, select Disk Utility in the macOS Utilities window,

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

How to create a bootable installer for macOS - Apple Support

You are not booting from a macOS 10.13 High Sierra installer, otherwise Disk Utility would have a "View" icon just above the left pane in Disk Utility. You must use the instructions in this Apple article for creating a bootable macOS USB installer since other methods usually do not work:

How to create a bootable installer for macOS - Apple Support

Plus, a system boot disk even for the installer will have macOS system files on the root of the USB stick and not just the installer app itself.

Also, installing & booting from an external drive over USB2 is going to be extremely slow and the Mac's performance will be terrible even with an SSD. If you have a Mac Pro tower, then you will get much better performance by installing another SSD internally either in one of the main drive bays or by converting the optical drive bay to support an SSD.

Right Sierra doesn't know about APOFS, but High Sierra does have APFS.

In the Disk Utility app  on your Mac, choose View > Show All Devices.

Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery, select Disk Utility in the macOS Utilities window, then click Continue. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data).

In the sidebar, select the top entry for a disk, then choose Erase.

What Format is the External SSD, Apple wants SSDs to be APFS, yet some older Macs require HFS+/MacOS Extended Journaled to get going.

But I thought you were usingDU from/ON High Sierra!???

Are you installing from a USB Flash Drive?

[Edited by Moderator]