It might be worth trying to reinstall via Recovery mode:I had the same issue. Trying to reinstall macOS in an old Mac whose hard drive died and replaced with SSD. Could not find the original CDs, nor a bootable copy of macOS to download. If I tried downloading with Mojave, I would get the error that the OS was not supported.
Apple Support said:Reinstall from macOS Recovery
Install the macOS that came with your Mac, or the closest version still available.
... If macOS Sierra 10.12.4 or later was never installed on your Mac, macOS Recovery works differently...
- Option-Command-R installs the macOS that came with your Mac, or the closest version still available.
- Shift-Option-Command-R isn't available.