There are cases where an answer will no longer be collected within a flow, but it still has dependencies (such as if you delete the question that collects the answer).
In that case, the answer will remain in the flow as an orphaned answer.
There are a two solutions to resolving orphaned answers
Create a new question that collects the same answer type and writes to the same answer key, and choose to re-associate it with the orphan.
Choose Add compatible question within the schema tab to create a question that will collect an answer of the correct type. Note that you may want to modify this auto-generated question to make more sense for the responder.