Can't install previous versions of programs for 2021, 2020, etc.

Click the button to Show Older Apps in CC Desktop App. 

Adobe wants users to have the latest apps, no more than 2 full versions back.   Going forward, if you must use older versions don't uninstall them.

Adobe now offers the current and ONE older version
.
I do not know how far back this process will go, but try...
Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process will
--only work until Adobe completely removes the old version(s) from the server
.
IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD