As Worldofneil said disc version can not be used with Steam version. They probably could make the text bigger or put in red text it does state it on the amazon page under product description.

This product is not compatible with the Steam version of FINAL FANTASY XIV: A Realm Reborn. If you’ve purchased FFXIV: A Realm Reborn on Steam, then you must also purchase FFXIV: Stormblood on Steam to upgrade your existing account.