Point of Sale API v1.3 includes improvements to API parameter naming and is available in the Square Point of Sale app version 4.59 and later.
merchant_id API request parameter is now
location_id. When used, the identifier for the business location that the Square Point of Sale application is logged in to must match the value of this parameter.
See the SquarePointOfSaleSDK-iOS GitHub repository for the easiest solution to add Point of Sale API to your iOS app.