Learn French: Elles ont sorti les fixations pour incliner le panneau. - They took out the mounts to tilt the panel.

Because here, sortir is used transitively, meaning it has a direct object (les fixations). When verbs of motion like sortir, monter, or passer take a direct object (you take something out, rather than you going out), they switch their auxiliary verb from être to avoir. If the sentence just meant 'They went out', it would be Elles sont sorties.

Since the subject is feminine plural (Elles), why isn't there an agreement on the past participle (sorties)?

Because the sentence uses the auxiliary verb avoir. With avoir, the past participle only agrees with a direct object if that object comes before the verb. Here, the direct object (les fixations) comes after the verb, so sorti does not change to match it or the subject.

Why is pour used before incliner?

In French, pour followed by an infinitive expresses purpose or intention, translating to 'in order to' in English. If you want to say you did something 'to [achieve a goal]', you must use pour before the infinitive verb.

What does the word fixations mean in this context?