Have you found figuring out how to the Group outputs for attributes in Blender5 confusing since the GUI was changed compared with previous versions? Just use the Store Named Attribute node directly wired between your main Geometry and the Group Input/Output.
Some older instructional videos for Blender4 show you UI interactions that are no longer supported exactly the same way in Blender5
In Blender5 you can easily create new named attributes on-the-fly (although it takes some practice figuring out how that works) and you can then prompt on it in other dialogs such as the Attribute dialog in the Shading Editor. But to get it to work properly, you MUST have your Store Named Attribute node fully wired up to/from the Geometry, otherwise it has no context for storing the new attribute name.