Shape armholes

I’m currently knitting the back part shaping armholes for a jumper and I am left with 68 sts. The next part says

Cast off 20 sts in patt, patt 27, cast off remember 20 sts in patt. Leave rem 28 sts on a stitch holder.

I have only 68 sts how is this possible to do?

This will work out although the directions are a bit awkward. Cast off 20sts. This will leave you with one stitch left on the right hand needle. Now work in pattern across an additional 27sts (28sts on the right hand needle), and now bind off 20sts.
There are other ways to do this but the above is according to your directions.
Can you give us a pattern link or a pattern name?

Thank you for your response the pattern is James c Brett jb241

Oh, very nice looking:
http://athenbys.com/JB241-James-C-Brett-Knitting-Pattern

If I follow what you have told me I have no remaining stitches to put onto a stitch holder :s

You should have the middle 28sts on the needle and available to put on the holder. Essentially you are binding off 20sts on either side of the center 28sts.
So when the pattern says to “patt 27” it means to knit those 27sts in whatever pattern stitch you are doing and let them sit on the right hand needle while you bind off the final 20sts.
You should end up with |||||||| this kind of look where the ___are bound off and the ||| are held sts. (20 + 28+ 20)