There is no function RegisterClassMap in the class Context so I searched for and find that it must be called "Configuration".
The full error message in Unity:
error CS1061: 'ReadingContext' does not contain a definition for 'RegisterClassMap' and no accessible extension method 'RegisterClassMap' accepting a first argument of type 'ReadingContext' could be found.
There is no function RegisterClassMap in the class Context so I searched for and find that it must be called "Configuration".
The full error message in Unity: error CS1061: 'ReadingContext' does not contain a definition for 'RegisterClassMap' and no accessible extension method 'RegisterClassMap' accepting a first argument of type 'ReadingContext' could be found.