I am trying to create a bootable macOS Mojave USB install drive.

Don't go to the Updates page. Just go to the Mojave page.

If you are going to just erase the drive and don't care what's on it, use Recovery to erase the drive and reinstall the OS. You don't need a bootable installer.

About macOS Recovery - Apple Support

Try the following to reinstall a fresh copy of the currently installed system.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (will be the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

Read Step 10 carefully. It's a selectable entry in the Utility Menu. It is all automatic.s