This inspection suggests that you use a C# 6.0 feature.
By default, ReSharper automatically detects C# version based on the associated compiler. However, you can specify the target C# version explicitly for a project — right-click the project in the Solution Explorer, choose Edit project item properties from the context menu and use the C# Language Level selector .
To set the C# version for all projects in your solution, specify it in a Directory.Build.props file in your solution directory as described here.
In .NET libraries, there are a lot of methods that are designed to accept names of types, members, or variables as arguments. One of the most common examples is the ArgumentNullException(string paramName). If you use string literals as the arguments for such methods, you can run into errors if the literal is misspelled or not changed when the corresponding symbol was renamed. A string literal that does not correspond to any valid symbol is not caught by the compiler.
The nameof operator, added in C# 6.0, addresses this — it allows capturing the string names of symbols that are in the scope.
In the example below, ReSharper suggests the replacement of the string literal "order" in the argument of the ArgumentNullException() with the nameof(order). The result is the same: nameof(order) returns the string 'order', but with nameof your code becomes less error-prone.