Some places in my pattern, ther is a black box who says knit a non existing stitch.

Need help with how to knit a non existing stitch... Thanks!

Well, it's hard to say without seeing the pattern, but sometimes a black box on the chart may represent a stitch that had been eliminated through a decrease on a previous row. In that case you would just ignore the nonexistant stitch. Does that apply for your pattern?

That's probably a 'no stitch' box and you just skip it the box; work the next st in the chart. It's like when a pattern says to 'knit 0' for a size.