The 21sts should be on a holder not the knitting needle. I’m guessing your beginning of round marker (BOR) is where it’s labeled below. The next 21sts (red line, a part of the back) are slipped to a holder and the new yarn end is started at the blue arrow before the second half of the back, the 24sts.
Then you knit around over the 24sts, the right sleeve, front and left sleeve. The 21sts on a holder are slipped off a holder and onto a needle. You may have to reorient the sts so that you can keep them from twisting this part of the back. Do this by slipping to another needle if necessary.