Wiktionary shows 2 usage examples for this verb. One matches the sentence in this exercise. But the other is "бояться за кого-либо." Can за be omitted from this example? And could за be inserted into the sentence in this exercise?
@RobertGeor16 - I just checked the Wiktionary entry and in this case it is not a good translation.
Бояться за кого-то means to fear for someone.
It's similar to how this works in English, since "we are not afraid of problems"
проблем is genitive plural. For "problem", the sentence would have to be "Мы не боимся проблемы." = "We are not afraid of the problem", with a genitive feminine singular ending.
Could страшно also be used in this sentence? For instance, my attempt: нас нет страшно проблем