The prohibited program message is known to trigger in phones that are rooted or jailbroken. If your phone is second hand and you are unaware of its own background, this may possibly be the case, as well.
As a last fix, try reinstalling MaplestoryM Mesos and relaunching it without any other programs running, even ones which are completely irrelevant to hacking. You might have an innocuous app that is somehow triggering the warning.
Failing all this, your very best bet is to get in contact Nexon right and report your issue. Since MapleStory M is newly launched on cellular, it is certain to get some hiccups here and there, and also the best method to prevent this from occurring would be to alert the developers quickly and politely.
If you are going to undertake each of the enemies and obstacles that'll get on your way in your experience in MapleStory M, you'll need to have some good gear on you. We are not only talking about the stuff you get your hands on at the beginning of the game, but you'll need to continue upgrading and getting better equipment in order to accept the harder tests in MapleStory M.
In order to find gear in MapleStory M, you are likely to buy Maplestory mobile mesos need to open plenty of treasure boxes. You will get one of them for free each day that you log-in, or you'll be able to spend 50,000 Meso to buy one. Alternatively, treasure boxes can be found in the game's Elite Dungeons, in case you feel confident about carrying them on.