How do I make the data to a nominal value, so the Learner can accept the data?
The answer depends on what your data is.
Can you upload the workflow and/or data so that we can have a look?
You normally need a string column. Do you have one in your dataset? Bag of word is a numerical matrix right?
If you have no nominal types (usually string types) try extracting your data from the document type with Document Data Extractor – KNIME Hub