Can I use InDesign (15.0)?

This is strange, the latest version is 18 so Creative Cloud should provide download only for 18 and 17. Anyhow version 15 means CC2020, for this version Adobe did not release any public download link, so you will have to contact Adobe support and ask them for the installer, If they have one available, they will provide you. To contact support follow the instructions given below
1. Go to https://helpx.adobe.com/contact.html?rghtup=autoOpen
2. Wait a bit on the page that opens up, it should automatically open a chat popup. If that does not happen then look for a call out/chat icon at the bottom right, click over it and the chat popup should open
3. In order to talk to a human type "AGENT" in the chat box
4. If you are not able to get to the chat window, then disable your adblockers, firewall, enable cookies/scripts on your browser. If it still fails then change your browser/machine/network.

P.S. Be mindful of answering anyone who sends you a private message. See the following for more information about scammers
https://tinyurl.com/y7cfrr7y
-Manan

No, but what is the reason that the team is not updating to a later version, as the current version is 18.x?