Flyff Legacy required good device specs to run smoothly, if your device is close or below the minimum requirements (iPhone 5S, Samsung Galaxy S4, or equivalent) it is likely the game won't run.
It is less likely to happen if you downloaded the device from the official stores, as you probably won't be able to download the game in the first place. But in case you downloaded Flyff Legacy from a 3rd party website, then you may face these technical issues.
Although we will do our best to offer Flyff Legacy on a wide range of devices, the game just won't work or the game experience won't be enjoyable if the hardware is not able to keep up.