Cannot implicitly convert type 'System.Data.EntityState' to 'System.Data.Entity.EntityState'. An explicit conversion exists (are you missing a cast?)

When you are using EF 6 or moving from EF 5 to EF 6, you should use System.Data.Entity.EntityState instead of System.Data.EntityState. This error happens when your project has reference to EF6 but you have code for EF5.

Also, check this link: When upgrading from EF5 to EF6 should I replace System.Data.Objects?.