You're trying to create a variations listing.
First thing: 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. (CDs do support 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 IS.
