Set a field mandatory on conditions to a pull-down values?
One answer
I don't think there is any out of box precondition for severity field. However you can try to create a script based condition and use the Required Attribute for Condition precondition for this.
http://www-01.ibm.com/support/knowledgecenter/SSCP65_5.0.2/com.ibm.team.workitem.doc/topics/t_configuring_dynamic_required_attributes.html
http://www-01.ibm.com/support/knowledgecenter/SSCP65_5.0.2/com.ibm.team.workitem.doc/topics/t_configuring_dynamic_required_attributes.html