While working with Leads in a Business Process Flow in Dynamics 365, I had a user decide they needed to edit some information on a recently Qualified Lead. They reopened this Lead, made their change and then tried to close it again only to be presented with the following error:
Upon inspecting the log file, I found this: Active stage is not on 'lead' entity.
After changing the active stage of the Business Process Flow to one that was on the Lead entity, we were able to requalify the Lead. Hopefully this helps you if you are ever stuck working with Opportunities, Leads and Business Process Flows.