Issue trying to run macOS Mojave off a bootable external USB
To run macOS off a USB drive, it needs to be installed on it first.

Unlike some Linux distributions, you cannot run macOS in live mode from the bootable installer USB. Also, it appears that you are trying to install macOS onto the same USB stick that you booted off. This is clearly not possible to do.

You can, however, connect a separate external USB stick/drive and install macOS onto it. Once installed, remove the USB stick containing the installer, start your Mac, press and hold Option key and boot off the external disk onto which you installed macOS.