My Document Properties window isn't appearing in Inkscape, even though I select it from the menu. Where is it?

If you can't see that window, you may have clicked the "Iconify this dock" button in the upper right corner. If you do that, it moves the window over to a line of vertical text at the right side of your screen. Go look for that text (circled in red in the image below) and click it to open it back up.