Trying to create Business Rules to make required attributes for a custom object and it is not showing the option:
Release : 16.1.1 and supported releases
This is as per the current design, the "Required action" attribute is only available for rules that can be created for Projects, Ideas, and Custom Investment Types and not designed for Custom Objects. Basically the action is only available for Objects with template attributes which enable the "New from Template" feature for creating instances on the object.
The use case would be an ideal enhancement candidate and needs a code change. Can be raised as an idea and prioritised via the Clarity Clarity Product innovation/ Enhancement process.