the weka libary differentiates between two types of ~nominal attributes. They call it a ‘string attribute’ if it has a unspecified number of values and a ‘nominal attribute’ if it has a known set of values (see, e.g., here).
This translates to KNIME to a String column without and with domain-values defined (see the ‘Spec’-tab in the table view). Thus, you can make the node work by applying the Domain Calculator node before (please note that the Domain Calculator by default won’t calculate the domain values if their number exceeds 60 - but that can be increased or disabled in the config dialog).