Can't install older version software OSX via partitions on High Sierra

I have done some digging via google and can't get a good answer.

I have obtained an iMac 2011 that came installed with HS. I partitioned the drive so I can install older OSX versions. I erased the partition and did it as Mac OS journaled. I can't get anything but HS to install. I get an error message when I try El Capitan or anything lower.

Is this not possible or is there another way to erase the disk space to take a previous OS?

Cheers!