High Sierra Installation Issue

[DuncanGarp]

Sorry if this has been asked before. I'm trying to install High Sierra into a separate partition from my main Mojave based partition.

I get this error:


Any way to bypass this? Thank you.

 

[CoastalOR]

Make a bootable USB installer for High Sierra, boot from the USB installer and then choose the target partition for the install.
https://support.apple.com/en-us/HT201372

 

[DuncanGarp]

Make a bootable USB installer for High Sierra, boot from the USB installer and then choose the target partition for the install.
https://support.apple.com/en-us/HT201372

Thanks again (2nd time you helped me specifically on this forum).
Awesome pic (walrus?) by the way.