Trying to extend the striped background to fill wall and ceiling

You could use the Vanishing Point filter. I would isolate the subject on a separate layer, and feather the top edge of he paneling. Then the clone stamp in the Vanishing Point will follow the perspective. I'd recommend using a small soft brush, working in sections, changing the sample point frequently to deal with the differences in lighting.

We sure do have to jump through some hoops nowadays. 😞

I needed the Gen Fill selection to overlap the model, so I copied the layer and used Remove Background, and then applied the mask leaving just the model.

Sure enough, when I used Gen Fill it decided that there was too much flesh, so Ctrl click the model cutout layer to load the selection , and fill that selection with black on background layer.

Use Gen Fill again — this time there is no flesh for it wory about, so it does what we need.

Turn the model cutout layer back on

Flatten layers > Job done

One of the hints that displayed when I tried generative fill recommended moving the subject to a new layer, then working on the background.

Hints rotate; I couldn't reproduce the message.

@jennifera82006732 to extend the background, your image needs more canvas space. 

A pop-up dialog box will open. 

So, if you want to add space to the top, then you need to anchor at the bottom as shown

If you want space all around, then you would anchor in the centre

Once you add the extra canvas to your image, you should be able to fill out the space nicely.