The input annotation feature classes do not use the same label engine. The Standard Labeling Engine and the Maplex Labeling Engine make use of different labeling placement parameters, and these parameters cannot be combined into one annotation feature class. To determine which labeling engine the feature-linked annotation feature class was created with, right-click the annotation feature class in ArcCatalog, click Properties, then click the Annotation tab. In the Feature Class section under Label Engine, it will list the label engine used when creating the annotation feature class.
Only append annotation feature classes that reference the same label engine.