The Greek Septuagint version of Amos 1:6 and 1:9 mention ‘captives of Solomon,’ however the Hebrew Masoretic text does not mention this little detail, only referring to captives in general. Nor does the Aramaic Peshitta. Is the Septuagint in error?
Well, the Bible records that both David and Solomon captured people from the surrounding gentile nations to use as soldiers and high officials. This includes Cherethites and Pelethites who served and fought for King David, and also Edomites who, despite being slaves, were given supervisory positions in his realm (see 2 Samuel 8:13-18).
Therefore, the Greek Septuagint version of Amos 1:6 and 1:9 could be the more accurate text, since it seems to refer to these particular peoples. However, we don’t know for sure.