Continuous: Set to Custom to be able to show a continuous axis by selecting Use continuous scale. Use continuous scale is only available for dimensions with numeric values. You cannot change sorting when you use a continuous scale.
We don't encourage changing error messages, though you can modify some of the generic ones as per How to customize the text of a generic Qlik Sense error message. Whether or not this message is among them, I cannot answer as this functionality is provided as is.