Do more usability testing

While still uncomfortable is when.

Do more usability testing. Do it earlier in the process. When you feel uncomfortable about it, because the thing is unfinished, that's exactly the time to do it. Very likely people stumble on something fundamental in one of the first steps.

You had a scenario that you wanted to test. This initial prototype may not work for how people expect to handle that kind of task. When that happens, shift to more research based interviewing, probing for stories about when people tried to do something similar before. This may lead to a point where you can pull up another screen relevant to that part of the story and explore from there.

This is likely a symptom of not having uncovered actual user needs in the first place. It happens. So forget about the prototype then and shift the focus of the conversation towards learning more about those wants and needs.