Errors in Keras Network Learner

when I use Keras network learner to create an image classification task, on the Keras network learner node an error appears like this:

“Error loading flow variables into node: The training data converter ‘org.knime.ip.dl.DLImgPlusValueToFloatTensorConverterFactory’ from network input ‘input_1_0:0’ could not be found. Are you missing the KNIME extension?”

how to solve this error?