Upgrading to High Sierra or newer on 2013 MacBook Air with external SSD and missing OEM internal SSD Background: I have a mid-2013 MBA I was given a couple of years ago without its SSD. I used it with an external USB 3.0 HD and upgraded the OS each new version up to Sierra, but could never get High Sierra to install.
Now I've just bought a cheap external USB 3.0 SSD to get a speed boost and I've installed a fresh OS via the Internet Recovery. It only allowed me to install Mountain Lion, which I guess was the current OS when the MBA was new.
I've gone to the App store to update to Mojave, the only version they seem to offer. I download and ran the update and when it's time to choose the drive to install on, my new SSD, it told me &quot;You may not install to this volume because the computer is missing a firmware partition.&quot;
One website said to install the firmware update from Apple's support site, but running it tells me I'm already up to date.
Some websites make it look like the only ways to solve similar problems requires the OEM SSD, which I have no access to.
I would prefer to have the latest OS but I'd settle for Sierra if it's utterly impossible to install anything newer. But I can only find Mojave on the App store.
(I do still have my external HD with Sierra installed but I thought a clean install would have some benefits.)

Output of diskutil list:
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *120.0 GB   disk1
   1:                        EFI                         209.7 MB   disk1s1
   2:                  Apple_HFS Mac USB SSD             119.2 GB   disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3
There are reports of other computers with a similar problem, and a damaged or missing internal drive seems to be a factor. 

From MacRumors:

"The EFI Firmware can only update using the EFI System Partition on the internal storage media. So even if you don't want to boot from it, you're still going to need a working drive in there to support firmware upgrades."

From Ask Different (this site):

"I have spoken with OWC customer service, and they say I will need to reinstall the OEM SSD. Then upgrade to High Sierra, which will update the Mac's firmware. Then re-install the OWC drive, and upgrade that to High Sierra."

So any system update that includes an EFI Firmware update will throw this error if you do not have an internal drive.

It looks like you will need to get a replacement SSD module for this Mac, which ultimately, is a better solution than running from an external.