What's wrong with "is connected to the shoulder"?
I see no problem with your translation except that the Duolingo computer probably saw the -ed of connected as past tense.
Does שריר have any meaning as an adjective in modern Hebrew, as הכל שריר וקים from the traditional ketubah text? I'm aware that ketubot are traditionally written in Aramaic but this phrase looks like it's Hebrew so i thought I'd ask
Well, the adjective שָׁרִיר means strong, but is not as much used as muscle: אני עומד לקבל התנגדות שרירה מכל הכיוונים "I am going to get strong resistance from all sides".
why is this not good?
this muscle is attached to the shoulder