"You must have Photoshop version 25.0 installed to access Generative Fill. Ensure that you’re using build m.2181 by clicking on Help in the Photoshop menu bar and then selecting System Info from the dropdown menu."
Are you first making a selection? To use it you must:
1.Use any selection tool to select an object or area in your image.
2.Select the Generative Fill button in the Contextual Task Bar that appears.
3.Write a prompt describing the object or scene you want to generate in the
text-entry prompt box. ...
4. Click Generate.