Merge fields are only available if the email data source is a Constituent List or an Imported List. For more information about data sources, please refer to What are the differences between the data sources Constituents, Imported List and None used for NetCommunity emails?

If you want to use a message that has a None data source with merge fields, you can take an email message and make a template from it --- be sure to specify the correct data source of Constituent List or the Imported List in the new template.