Welcome to TalkGraphics
I see that you have solved the problem. But others may have the same question.
The page background is locked.
Open the Page and Layer Gallery (F10). There will be a locked padlock icon next to the Page Background Layer. Click the padlock so it is in the open position.
Edit your page background. Then lock the layer again.
If you don't lock the layer, it can accidentally get selected and moved.
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