You do retrieve a Event by association. As far as I can see this microflow doesn't have a input parameter. Thus you retrieve an event over association by the new create EventSignup.
Which will result in an empty variable, since the new object will have a empty reference.
My guess is that you want to signup for an event as selected from a list or from the details of a single event.
NOTE: this is only possible if you have a Event in context when triggering the microflow. If you trigger this form a button or navigation item without having a event object available, this won't work. But then you have to sortout to which event this signup is made.