I have been having this issue lately when trying to create complex rules. I get to the end of my rule and I have Ended the sub rule but then there isn't a selection to End the Rule. I usually have to Erase the Rule and start over and then sometimes it will be in the drop down list and sometimes it won't. I have done this rule twice now and it is still not showing so I can't complete the rule.
It did let me complete the rule actually this time but sometimes it won't.