This is a `to be` phrase. Here in place of `to` we use subject (nouns and pronouns) and in place of be we use helping verbs like am, is, are, was, were, will be, shall be etc.
`To be in a fix` means to be in a problem. We hear people saying `fix him` that means do not allow to move. Example of phrase can be `I am in a fix as my friend who took money from me has not been returning it`. OR `my friend tried to save life of the person who met with an accident on the road, however, now he is in a fix as he has to spend time facing interrogation of police`.
So, take care, do not be in a fix or try to avoid to be in a fix, also never put any of your friend in a fix.
Enjoy the skill of idioms and phrases and enrich your English.