Variation listings require items to be in the same physical condition.
Generally speaking, you'll want to group by something that ties items together. For example, all Harry Potter on one listing, or all Agatha Christie on another listing, or all science-fiction on another listing. A hodge podge of random books on the same listing will make it hard to find in search for the buyers looking to purchase those items.
Also, you cannot set up different shipping rates for different variations so all books on the listing should be similar size/weight.
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Variations are not supported on the app. Make sure you're using the website. Variations are not supported for auction format listings; make sure you're using fixed price.
By default the variation setting is hidden on the listing form. Once you unhide the setting it will stick for future listings.
On the top right of the page click the settings menu (3 vertical dots) and toggle the variations slider if it's not already on.
Make sure you're working off a new listing, not a relist or sell similar from a listing without variations.
Beyond that it comes down to category; not every category supports variations.
If the category supports variations you'll find them in a non-intuitive, awkward placement on the listing form - below item specifics instead of above them. Make sure you set up variations before you fill out any of the item specifics.
