Illustrator does not work with iCloud Drive enabled in Mojave OS X 10.14
Since upgrading to Mojave OS X 10.14, Illustrator does not work with iCloud Drive enabled. Will not open or save files.
Illustrator does not support network or removable drives as iCloud, scratch disks, or folders of file sync services such as Dropbox and Google Drive.
Pleas refer to ,pre information here: https://helpx.adobe.com/illustrator/kb/illustrator-support-networks-removable-media.html