Unable to Preorder S24 through Samsung

This was also an issue last year.  I ended up giving up and just ordering through T-Mobile last year.  There was a fairly large thread of people having issues trying to complete orders through Samsung that wanted to purchase a device through T-Mobile's EIP that wanted to take advantage of Samsung's accessory credits.  If you really want your device sooner than later.  I would personally just give in and order it through T-Mobile.   It has something to do with how the system is linked to add it to your account.  Direct orders through Samsung for the Unlocked version don't have any issues trying to place an order so it's more than likely T-mobile's system not being able to deal with demand.