CS0234 The type or namespace name does not exist in the namespace (are you missing an assembly reference?)

The description of the message informs you which is the missing assembly reference. In my case it was System.Data

This is how you fix the assembly reference-



Look at the error -> my error was "The type or namespace name 'Entity' doesn't exist in the namespace System.Data.
It means it is asking me to add System.Data.Entity library explicitly
Go to your project
Right click on References
Add Reference -> in my case it was System.Data.Entity