There is not pressure-sensitive stylus support for the iPhone version of Sketch.
Hope that helps.
Hi Sue, thanks for your reply.
Just to be clear, you're saying that Adobe Sketch won't recognise the Jot Script 2 stylus on iPhone, but might work on iPad?
It won't work on iPad Pro (only Apple's Pencil works on that), but you should be okay on other iPads. (Here's an FAQ; the third question addresses pressure-sensitive pens: Photoshop Help | Adobe Photoshop Sketch FAQ).
By the way, I have a Jot Touch 4 and a Jot Touch Pixelpoint and they're both great on my iPad Air 2.