Older version of Indesign?

Adobe assumes you have modern equipment and operating system, no more than 2 versions back.

Most Creative Cloud apps are built to work on:

Windows 11, Windows 10 versions 21H1, 20H2, 2004, and 1909 (LTSB branches are not supported);

macOS 12 (Monterey), macOS 11 (Big Sur), macOS 10.16 or 10.15 (Catalina)
- https://helpx.adobe.com/creative-cloud/system-requirements.html

Whats wrong, why can't I download an older version?

In short, what the others said.

In detail:

• The two versions in the Cloud desktop app require macOS Catalina or later
• The latest OS version your iMac can run is OSX High Sierra so you are not offered any versions in "Other versions" since none are compatible with your operating system

What you can do is:

• Upgrade to High Sierra (if you're not already running it).
• Contact Adobe to get the offline installer for InDesign 2020 which is the last version compatible with High Sierra
  For full steps, see https://prodesigntools.com/adobe-cc-2020-direct-download-links.html

You can check with Apple Support whether you can Upgrade to Mac OS High Seirra.

Better to plan Upgrade with Apple Support advise and based on other apps installed on iMac and its compatibility for newer version Mac OS

https://support.apple.com/kb/sp765?locale=en_US

I dont think iMac 2011 can go beyond , High Seirra. Even for Mojave , its requires iMac 2012 or newer Hardware 

https://support.apple.com/kb/SP777?locale=en_US

Only the current version and most recent past version are available. That is a very old machine but you should be able to get it updated to a reasably recent version of Mac OS are decent. But even then you're going to be limited with an 11 year old processor and a lack of RAM.