You are trying to select a picklist value on the New Comment Page but it is greyed out or only showing one value.

The Summary section of the New Comment Page is intended to be a read-only section of key fields of the case, such as Subject and Description. The Edit Fields section is where you can have fields to be edited while agents compose new comments.

While adding fields to both packaged field sets (or custom), keep in mind that the Summary section cannot contain controlling picklist fields.

See an example in the image below, where the Case Type is the controlling field and "TESTING" is the dependent field. The Edit Fields section displays as read-only when the controlling field is also in the Summary section. 




Solution: remove the controlling field from the Summary section and have them only in the Edit Fields section.

This is a Salesforce platform limitation.