Process Model

The icon ion the User Input Task node AND the error message are both telling you that you need to assign this node to someone so that they can work the step. Think on a User Input Task as a "service request!" but instead of a "silicon-based REST service" it's calling a "carbon-based service". But in both cases you need a defined end-point. In this case that end-point is configured by setting the Assignment. This can be to a defined user-account or a Group.