1144: Interface method addChild in namespace mx.core:IChildList is implemented with an incompatible signature in class.
Method signatures are not matching. Method signatures must match exactly.
Compare the class to the interface. If a method is public you cannot use private. If a value is of type String the value and return type must be type String. Override must be specified.
The project you are working in is using a mobile SDK and you have a reference to another library that is using an incompatible SDK. This happens when an mx namespace is used or referenced in a mobile spark project.
You must remove the library that is incompatible and create or reference a library with a compatible SDK.