Generative Fill function is not available

[tefisen pad printer]

I am looking for help with a few quick tips on the generative fill faunction. I want to automatically fill the left and right sides (blue area) of this picture to match the background of the main picture. I heard that this generative filling can be done easily, but this item is gray. How can you use this function?

[AxelMatt]

Which image mode to you use for this picture?

If check this for 16bit/channel (and 8bit/channel) RGB Color and the Generative Fill is available.