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I have got two alignment tool options here side by side, so you can see all of the menu. You cannot see the alignment tools once the relative menu is open but once you choose what you want it will be visible
There are 6 things to choose from to set your alignment relative too.
I opened a background and then a smaller off centre shape. I selected the smaller shape, set the alignment to 'first item' then chose the alignment buttons to make it central. I tested them all and if you want a layer above the background or image aligned, check relative to first item.