I still have an issue with my iMac. I had to install a new harddrive because the old one was brocken. But doesn't matter which version of iMac install DVD I choose it always "Mac OS cannot be installed on this computer". I also reseted a view times the NVRAM & PRAM but still the same result. It only gives me the option to restart or to restore but as I said the old HDD is completly broken.
You can refer to this article on how to do that. https://everymac.com/mac-identification ... ation.html
Knowing which iMac can help solve the issue.