Check out the patch notes for the v20 Lovely Update here:

Cannot Move In-game in Specific Missions Bug

MapleStory 2 Rep: 4,030
Posts: 320
in Bug Reporting
Hi Maplers,

There is a new known issue since the Skilled Ally update where in certain solo sequences, you may find that your character is unable to move. This is most common in the Maple Guide for mounts as well as specific story scenario missions. As a temporary measure to bypass this bug, you can use your emotes (F1, F2, etc) to unstuck and continue the mission.