Possible meanings are 1) Paul is commanding the Galatians not to trouble him, "I am commanding you this: do not trouble me," or 2) Paul is telling the Galatians that he is commanding all people not to trouble him, "I am commanding everyone this: do not trouble me," or 3) Paul is expressing a desire, "I do not want anyone to trouble me."
These marks were scars from people who beat and whipped Paul because they did not like him teaching about Jesus. Alternate translation: "for the scars on my body show that I serve Jesus"