When downloading the Adobe Illustrator extension directly (rather than using the Creative Cloud tool) ArcGIS® Maps for Adobe® Creative Cloud®, the filename is: "ArcGIS®_Maps_for_Adobe®_Creative_Cloud®_1.2.0.zxp".
If you import this extension file using the Adobe Manage Extensions Utility, the import fails and a reason is not given. It turns out that the special characters (®) in the filename prohibit a successful install. If these characters are removed from the filename, users can install the extension without errors using tools other than the CC tool. This is the case on many on-premise deployments.