The pattern set-up rows should read alternating K11, P11 rather than all knit and all purl. So, when you get to row 15, switch this sequence around so you’re purling on top of the knitted squares, and knitting on top of the purled ones (coming out as stocking stitch and reverse stocking stitch).
If you'd like this clarified at all, email knittoday@originpublishing.co.uk

