Dynamo no longer supports ProtoInterface. Users need to remove ProtoInterface references and include references to DynamoServices. You may experience missing directive or assembly reference error due to this change.

Under Solution Explorer > Expand Your Project > Expand References, you will see a yellow triangle beside ProtoInterface which indicates that the dll could not be found.

You will need to remove the reference by Right-click on ProtoInterface > Remove Reference.
You can download/update the latest DynamoVisualProgramming.ZeroTouchLibrary nuget package.
It will automatically download the necessary binaries which includes DynamoServices.