Are you looking on DL.com for the GOT-AMS segment only? If you search for it as a standalone intra-EU flight it will not show up on DL's site (it will tell you that DL does not operate flights between those cities). This is true even if the actual flight is a DL codeshare - the codeshare is only visible and can only be booked if combined with a TATL segment. If you search GOT-AMS-US, it should show up on DL.com if still a valid flight.