System.Runtime.Serialization.DataContractAttribute doesn't exist in the namespace System.Runtime.Serialization

I'm doing some WCF work and I don't see why I run into this error.

System.Runtime.Serialization.DataContractAttribute doesn't exist in the namespace

System.Runtime.Serialization

The microsoft documentation clearly shows this hierarchy

System.Object
  System.Attribute
    System.Runtime.Serialization.ContractNamespaceAttribute

Anyone knows what is going on?