I am building an app using sketch and the client wants all mobile phones including Iphone and android to work. How do you export the screens to work on all Iphones and android.
Sketch can't automatically recreate one single screen to accommodate multi-platforms (Android, iOS, Windows Phone, etc), or multi-resolutions.
The platform limitation has more to do with the inherent differences in each platform - a UX/UI/Product designer typically has to make very specific considerations for how their app behaves on each platform. E.g. a "dropdown menu" behaves different in iOS9 than it does in Marshmallow (assuming we're talking about native components for a moment). Sketch can't determine what your experience should look or behave like across all platforms - that's going to take purposeful design.
My recommendation would be to tackle discrete experiences / user journeys across both platforms at a time, instead of designing either app in a platform vacuum. E.g. design and test your "sign in / register" flow in iOS and Android, then move on to your "Create new [fill in the blank]" flow. This may help you keep your experiences familiar enough that a user who picks up the app on both platforms isn't completely surprised by either experience.
If you're app's experience and interfaces are largely "custom", i.e. not heavily reliant on either iOS's UI guidelines or the Material Design standards, then you may be able to get away with designing your app's flows just once, and presenting them to your client in platform specific shells. Apps like InVision make it easy to take one set of screens and present them in either an iPhone or Android shell, depending on your audience.
Hope that helps!