If you can’t install macOS on an external disk
You may need to repartition the disk because it may not have the partition scheme required. You can install macOS only on disks with the GUID Partition Table scheme. You can use Disk Utility to check the external disk’s partition scheme.
Important: If you need to repartition your disk, first back up any important data.
In the Disk Utility app on your Mac, select the disk in the list on the left.
Note: If you see a window explaining how to create new volumes using APFS, click Partition in the window.
Look at the Partition Map entry in the pane. If the entry doesn’t say GUID Partition Map, repartition the disk.
If you don’t see the Partition Map entry in the pane, you may have selected the volume instead of the disk. (Disks are flush left, with their volumes indented below them.)
If you partition using the APFS format, your Mac must have macOS High Sierra or later to access the drive. Earlier versions of the Mac operating system don’t mount APFS-formatted volumes. If you have Mac computers that have an earlier version of macOS, partition using the Mac OS Extended format so that your other Mac computers can continue accessing the drive. After the disk is partitioned, you can install macOS, then restore any data you backed up. For more information about choosing a disk format, see How to erase a disk for Mac.
Important: If you have an iMac Pro, the Startup Security Utility provides security features that give you control over how your iMac Pro starts up. On other Mac computers, the utility provides firmware password protection for Mac computers that support use of a firmware password. For more information about the Startup Security Utility, see What is the Startup Security Utility?