I installed a free trial and when it expired, I tried to input the product serial code that I got but I can't find where to put it. It seems that it only let me buy subscription for the creative cloud. But I already got a product serial code, just can't figure out where to put it and if I need to uninstall the creative cloud first. Any help is greatly appreciated!
You don't need a serial for CC-versions. Need to have Adobe ID (email+password).
If you have license for old version than you need to have official distributive that you can find in your Adobe account in this page - licensing.adobe.com. When you will install, installer will ask you a serial. Or you try to find in help menu - some as "activate"
What version of InDesign is your serial number for?