Q: I have OS X El Capitan version 10.11.6 and want to download Sierra and cannot do it

Is your Mac compatible?
Check it here: How to get macOS Sierra - Apple Support

The Sierra drivers do not support the hardware in your 2008 iMac. This is why it is not compatible. So, regardless if you have enough disk space, RAM and CPU power, your hardware is out dated for macOS Sierra and the highest supported OS is 10.11.6 (El Capitan).

For an iMac, you need as least iMac (Late 2009 or newer) to upgrade to Sierra, really sorry to say that you have to get another iMac if you want to run Sierra.