iMac 21.5" 2017 Late 2012 Won't upgrade beyond Sierra due to missing firmware partition?

Internal HD has died so booting off external SSD 2tb.

How is the external SSD partitioned/formatted? It should be APFS for macOS 10.13 or higher.

File system formats available in Disk Utility on Mac - Apple Support

Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

Using Safari, download and install the 10.12.6 Combo Update,

at > https://support.apple.com/kb/dl1931?locale=en_US

Then using Safari, see if you can upgrade to High Sierra using the macOS High Sierra 10.13 link,

from > https://support.apple.com/en-us/HT211683

Unfortunately you need a working internal drive with a properly partitioned & formatted drive in order to install macOS 10.13+ for the first time since the system firmware included in the installer requires it. I have confirmed this requirement several years ago when I first encountered the problem. It is a shame that Apple does not allow the system firmware updater to use the external drive especially when the OS is being installed to the external drive.