。
She was afraid of traveling alone.
Literal
She [topic-は] solo-trip [subject-が] was-frightening.
一人旅 ('one-person trip') is a fixed compound that names solo travel as a category — heavily romanticized in Japanese travel essays, NHK documentaries, and 'find yourself' narratives. が marks 一人旅 because 恐ろしい ('frightening') is an i-adjective of feeling: the source of the emotion takes が, while the experiencer is the topic. Past 恐ろしかった ties the fear to a specific past situation — perhaps before an actual trip — rather than a general disposition.