You can’t use an external CD/DVD drive connected to a NUC running ROCK. The ROCK operating system is designed and built solely for the purpose of running a Roon Core, and nothing else.
At the moment, you’ll have to use a PC or Mac with a drive to rip CDs. @Jan_Koudijs is working on a CD Ripper extension for Roon, but at the moment ROCK does not host extensions, they need to be hosted elsewhere, e.g. on a RPi running Linux.