When the Summary Attribute input port is used, the Merge Features operator does not output any geometry. The Features Output operator fails with an error: "Input Features have no geometry".
If the input features have geometry and if the geometry output is desired in the output features, then a summary attribute defining the output geometry must be provided. Attributes have data types (for example: number, text, date, etc.) and some attributes are of the data type “area”, “line”, “point”, etc. Through the eyes of the Merge Features and Generate Functional Attributes operators, geometry is simply another attribute of a particular type.
Please follow these steps to resolve this issue:
Right-click on the Merge Features operator to add another SummaryAttribute input port.
Double-click the new SummaryAttribute input port.
Give an output attribute name of “Geometry” (or anything else).
Enter the expression “MERGE(Input.Geometry)”, substituting “Geometry” with the actual name of the input geometry field which you can see on the expression dialog.