You could buy a 64 GB card and have the same message. The message is about the internal storage, not the SD card storage. Most apps and app data do not automatically save to the SD card (for a variety of reasons). This is not an issue with the phone (other than the fact that it's a budget phone and all the downsides that entails), as it's only made to do so much. If you are rooted, there are some options, possibly, such as Links2SD, swapping the mounting points to switch the internal and external storages, or something like one of the CronMod solutions. If you aren't rooted or don't want to, then your options are severely limited to basically moving some of the apps to SD, if supported, but it's not a great solution as it does not move much of the data over and doesn't support all apps, meaning don't put keyboards, live wallpapers, home launchers, or widgets on the SD card.