Why can’t I change author name?

I'm not sure what device you are working on, but assume for a moment that you are on iOS.

You'll find the option to change the author in the app.

I'll try to take you there step by step.

1. Open Pages with, for example, an empty document

2. Click on the circle with the three dots at the top right

3. Click on settings (with the wrench)

4. Next you will see 'author name' click here

You can change the name of the author here. Then click 'done' at the top right.

I hope that this works.