Why this error occurs?

I just used GetWordTimerManager().SetTimer() and it says that it is an incomplete type that i used.
Why this error occurs? And how can I fix this?
(I’m korean, so my English can be little awkward)

When you see errors like these (incomplete type), it means you’ll need to include the relevant header file

If you use the Visual Assist X plugin, right click on the SetTimer function and select “Add include”. Otherwise, you’ll have to manually find where the definition of GetWorldTimerManager is stored and include that file