I’m sorry, but I’m not confident of her success.
Very unnatural robotic translation… Here are my alternatives:
確信する itself is a frequent phrase, but 彼女の成功を確信していません can be interpreted as “I do not wish her success”. The original English simply says that the probability of success is unpredictable, but the original Japanese implies that I bear a personal grudge against her.
Please be noticed that mookeee, a native Japanese speaker on Tatoeba, adopted many sentenced imported from Tanaka Corpus, but I often found their adopted ones unnatural.