static_cast (or C-style cast) is almost always(*) the wrong approach! The error explicitely states that the type being used is not compatible to the type being expected, and that is a sure indication that you need an actual conversion function, not a type cast!
In case of CStrings, what you need is checking the available functions and using them properly. As indicated here
], if strSource is of type CString, you should use the GetString() method on it, not cast it to a type that it is not compatible with.
(*): The exception is using a type cast to PCXSTR, because that cast is overridden in the CString class to implicitly perform the required conversion.