<< Compatible internal storage is required in order to update >>
Obtaining the manual firmware update will not solve this problem.
You need to do what it says -- install a compatible [genuine Apple] internal drive, and run the software update.
NOTE: it is the firmware Installer that is stored temporarily on the compatible drive. The firmware itself is updated into a private store on the processor card, and is not permanently stored on ANY drive.
so go back to the source of the update card, or to eBay, and obtain a working Apple original SSD for your computer.
Run the software update to the version you desire. The firmware update will be installed in that process.
Swap back to your old drive, and the firmware update will no longer be required (already installed).
Perform the software update (on your long-term drive) to the version you desire.