On my Mac, deleting spots with the delete button does not work. My Mac OS is Mac OS Sierra.

I am working with Delta2D on a Mac. Since I upgraded to MacOS Sierra (Mac OS 10.12), deleting Spots in Delta2D does not work any more with the keyboard shortcut it used to before: [fn] + [<-]. What can I do?

Background of your problem is a problem between Java 8 and MacOS Sierra:
https://java.com/en/download/faq/sierra_java.xml

Meanwhile (as of December 15th, 2016) this problem is fixed by an update of Mac OS Sierra. Please update your Mac to version 10.12.2.