A fragment is a sentence missing its subject or main verb.
Now let's look at the two sentences given and identify the subject and/or the main verb:
Sentence 1: Lena Adams Foster's teammates told her that their camouflage uniforms would never work.
The subject of the sentence is Lena Adams Foster and how her teammates thought that the uniform would never work.
Sentence 2: Especially because they accidentally used glow-in-the-dark thread.
The subject of the sentence is unclear all we know is that someone used glow-in-the-dark thread in something that is not known to us based on this one sentence.
Sentence 2 is a fragment because the subject and/or the main verb are not present.
