Disk Utility won't erase a CD.

That appears to be the case, for now. Disk Utility got a complete overhaul in both design and functionality in El Capitan. Some features have yet to make a return, some are more difficult to use, and one doesn't even make sense. That last one is having the ability to partition a drive under the Erase button instead of Partition. Like that makes sense to anyone.

I tested a CD-RW disk under Yosemite. With the older and mature version of Disk Utility, there's no problem erasing such a disk. In El Capitan, it can't be done. We'll have to wait until Apple gets done baking the new version.

thimbleberrybumblebee wrote:

The disc is a DC-RW.

cf. Kurt Langs remarks above.

Always good to send Apple Feedback http://www.apple.com/feedback/

I don't have a RW disk to test on but Control (right) - click on the disk icon on the Desktop and see if the erase function is in the contextual menu. That's where the burn option is located:

Did you use CD-RW media or CD-ROM media?

The most common CD is not erasable.