Keras ImageDataGenerator flow_from_dataframe : Found 0 validated image filenames belonging to 0 classes

I am working on 3D image data stored as .npy files with shape (128,128,128). When I try to use Keras ImageDataGenerator it throws the following error:

Found 0 validated image filenames belonging to 0 classes.

My code is as follows:

processed_data =  r'C:/Users/dush/Desktop/Project/preprocessed'


from tensorflow.keras.preprocessing.image import ImageDataGenerator

img_size = 64
batch_size = 32

data_gen = ImageDataGenerator(horizontal_flip = True,
                              validation_split=0.2,
                               fill_mode = "nearest",
                               zoom_range = 0.3,
                               width_shift_range = 0.1,
                               height_shift_range = 0.1,
                               rotation_range = 30)


train_gen = data_gen.flow_from_dataframe(
        dataframe = train,
        directory = processed_data, 
        x_col = 'id',
        y_col = 'category',  
        target_size=(img_size, img_size, img_size),
        batch_size = batch_size,
        class_mode="binary",
        #validate_filenames=False,
        subset='training', seed = 23) #image_generator.flow_from_dataframe

valid_gen = data_gen.flow_from_dataframe(
    dataframe = train,
    directory = processed_data, 
    x_col = 'id',
    y_col = 'category',
    target_size=(img_size, img_size, img_size),
    #validate_filenames=False,
    batch_size=batch_size,
    class_mode="binary",
    subset='validation', shuffle=False, seed=23) 

If I set validate_filenames=False, then it identifies the filenames but get an error when fitting the model. What am I doing wrong?