The output of “TrainVectorClassifier” application is a model file that you can use with the “VectorClassifier” application. I don’t think you will be able to use it with Model Designer of QGIS.
When using the “Segmentation” application, you can select the segmentation algorithm with the -filter [meanshift|cc|watershed|mprofiles] parameter. If you select the cc (Connected Components) algorithm, you are expected to provide the -filter.cc.expr string parameter, to precise the connection condition for the algorithm, written as a mathematical expression. For example, you could set this parameter to intensity_p2 > 0.5.
Your error means that the mathematical expression provided is not built correctly.