can't install Mac OSX 10.13.6 on external hard drive

I suspect you need to format the target disk.

Drive Preparation - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list. 
3. Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

You must not format the target disk using APFS because High Sierra cannot use it as a bootable disk.

The disk you have highlighted is the USB installer so it is not a valid destination. Follow @Kappy's instructions to properly erase your external drive before you select the "Reinstall macOS" option.

FWIW, you should have created the High Sierra Bootable Installer to a USB flash drive, boot from that then install High Sierra to your external drive.

I didn't consider that the OP may have used their external destination hard drive as the installer. Good catch.

I try 😎

You're welcome. It is a bit odd but stuff happens I suppose. :)