A shield? That's called a cable. Looks like the translator (or maybe even the original author) doesn't know anything about guitars.
Original text literally used the the word shield, 「シ…シールドが… 挿さって…ません…」, which is why I went with it.
Maybe I could've changed the wording of this phrase better, but just like you mentioned, my knowledge on guitars is tiny.
On a lighter note, the song "We're off on a secret mission" kept playing in my head whenever Ako, Rinko, and Sayo interacted.
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