Symptoms

Create Change Request UI Actions are not available on an Incident/Task form's right click header menu

Release

All releases

Incident/Task record already has a Change Request attached to it in the task.rfc field on the form
As a result, and as per the condition in the out of box Create (Normal, Standard, Emergency) Change UI Actions the ui action will not be available on the form

Assuming a new Change Request needs to be created for the record and to have the Change UI Actions available on the form header menu, clear out the existing Change Request value in the task.rfc field and save that value as a Work Note or Additional Comment for future reference and save the form.
This should allow the Create Change Request UI Actions to be available on the form's header menu (for Task records such as Incident, Problem etc) when you right click on the form.