Check out the patch notes for the v17 Precursor Update here:

Cannot Move In-game in Specific Missions Bug

MapleStory 2 Rep: 3,270
Posts: 247
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.