Can anyone help. I have Mavic XM319 rims and New Nobby Nic tubeless easy tyres and a Stans kit which claims to cove 17-19mm rims from 26 to 29. So why is Stans rubber rim tape not able to sit below the rim both sides? And what can I do to get a good seal?

You don’t need rim strips with that setup, you need rim tape. The tyres and rims will do the rest, although there are tales of Schwalbe and Stans being a pretty tight fit.