When you create an order via Mobile Sales App you must have a customer to add to that order. Creating a new customer on the go only takes a few mins with the ridiculously good Mobile Sales App.
First, go to the Relationship section of the App.
Touch the New button on the top left-hand corner of the page.
Once you've filled out all the details for your new customer, hit "Save" and a new Relationship will be created. That's it, so simple!