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.
Open Disk Utility, then select the disk in the list on the left.
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.)
After the disk is partitioned, you can install macOS, then restore any data you backed up.