I want to delete a group/pen, but I cannot select it.

The group/pen is in use by another part of Velos. A group/pen can only be removed if it is:

See the icon https://velos-online-help.azurewebsites.net/en-US/cows-webhelp/images/Icon%20exclamation%20mark_633.png to see if the group/pen is in use.