Is it possible for guinea pigs to be afraid of climbing a ladder?

Answer to your question. Is it possible for humans to be scared of ladders? (answer, yes for I am). Animals can have the same fears we do. If she still doesn't want to go up despite the fact that the others are, she clearly doesn't want to. Give her time, and if she still doesn't, if curiousity doesn't win, yes, she is scared. Let her do this her own way.