Welcome to KH!
You started with 26sts for the heel and you should end with 16sts. You should be able to work 10 rows with one dec in each row (6 more rows after the 4 rows given in the pattern). a gap forms where you turn the work and you’re knitting or purling until one stitch before the gap then knitting or purling together the stitches on either side of the gap.
Maybe give it another try or try it again with scrap yarn.
You can see the gap (~1.55min) in this video although the instructions for this heel are slightly different.