Word applies page setups on a section by section basis. So in order to vary the page setup setting such as Portrait/Landscape you need a section break at the point where the page setup changes. Once that section break is there, you can change either section independently of each other.

If you no longer needed the variation in the page setup, you could delete the section break and the first section will inherit the page setup of the second section.

It can be hard to see section breaks in page layout view. If you change to Draft view, it is much easier to see and delete section breaks.

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Andrew Lockton
Chrysalis Design, Melbourne Australia