In Pages, if you send an object to the back of the page from the Arrange menu, you may not be able to select it by simply clicking on the object. It may appear behind the document's text or another object.
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If you need to select the object to edit it:
Move the arrow anywhere outside the text areas of the page and then click. The insertion point should not be visible.
Drag the cursor onto the object you wish to modify. If additional items are selected, hold the Shift key down and click the items that you want to deselect (Shift-click).