My problem is do not turn work.
Up to this point you have been knitting flat i.e, knitting both sides of the piece (regardless of whether you have been using straight needles or circulars). The pattern states to work the last (14th) row using a circular needle as, from here on, you will be working in the round - this is why it says ‘Do not turn work’ (from the ws to the rs, you continue on with the back and purl st)
Hope this helps
It’s a bit like knitting the tab for a shawl. You’ll knit a small rectangle ending on the WS and without turning to the RS, pick up sts along a side edge then along the cast on edge and finally along the 4th edge.
This video is similar although sts are only picked up on 3 edges.