Can't select a disk to install High Sierra
I think the installer is not allowing you to install because you need to FIRST update the firmware.  You do this by making a USB installer from the installer app and booting to it.
https://support.apple.com/en-us/HT201372

You will need to choose HFS+ as the format and the High Sierra Installer will convert the disk to APFS (if the target drive is an SSD).