I cannot install older verions of After Effects on my (late 2013) iMac - High Sierra

My iMac  (Late 2013, 64bit  - 3.1 GHz Intel Core i7) is running High Sierra and it seems that it cannot update to newer OS'. That's fine. However, The newest versions of the Creative Apps don't work with it and so I'm installing older versions (photoshop & aftereffects)

Here is the beginning of the problem: I am trying to install after effects; I am choosing the 'other versions' dropbox, and selecting an older version. All of the buttons are not responsive. Actually, I just tried again, and even the most recent version's install button is unresponsive. 

What I've tried:

• Repairing the Creative Cloud Desktop install

That's the only thing I've tried because after that everything got worse. I can't open the creative cloud anymore - when I do I find that the text and images don't load, only a graphic peach colored shape - and when I go to uninstall the app entirely I get a message about other apps depending on the cloud, so it cannot be uninstalled.

Please help me fix this. Since I need this for work, the options I'm considering are changing to other software like Nuke, or getting a new computer.