First step of debugging: If you add a Print node to your input call, does the message get displayed?
If your input handling is in the Player Controller, your character probably gets spawned with the wrong (or no) controller. To fix this, open your Character Blueprint’s Class Defaults and check whether the correct controller is assigned. You’ll probably also want to change the Auto Possess AI setting with “Placed in World or Spawned”.
If your input handling is in the character blueprint, the most likely reason is that your input is blocked by the Player Controller. To fix this, open your Player Controller Blueprint’s Class Defaults and set Block Input to false:
A picture of your bpwould be great. Also unless your working with paper2d you do realize up wont just move the character up? Its a 3d world, theres gravity to contend with etc.
If the print string is spamming without the button being presssed you put it in the wrong place so a picture would be great. Ctrl+alt+printscreen with unreal engine 4 open and then paste ctrl+v at imgur