Can't declare uniquely named class.. because it's "in use"
For PS4 class autodiscovery to work, your file name needs to be the same as your class name. From reading the error message, it appears that your file name is TransformLabels.php, but you've named your class transform_labels. You need to change your class name to TransformLabels:

class TransformLabels extends ProcessorPluginBase implements BuildProcessorInterface {


One other question I have is, did you create the facets processor plugin type? Or was it a 3rd party?