I had the exact same problem, unfortunately, adding the primary key doesn't solve the issue. So here's how I solve mine:
primary key
on the table so I alter my table and add a primary key.Delete the ADO.NET Entity Data Model
(edmx file) where I use to map and connect with my database.Add again a new file of ADO.NET Entity Data Model
to connect with my database and for mapping my model properties.Clean and rebuild the solution.
Problem solved.