Hi Felipe,
In order to approve purchase order, you don't need the SAP Mobile Platform.
You need to install and configure the "Approve Purchase Order" fiori application.
You can find more information in the following link:
Approve Purchase Orders - SAP Fiori principal apps 1.0 for SAP ERP - SAP Library
In order to use this app, you need to install some components in the SAP ECC backend, and the SAP netweaver gateway that can be installed in the same ECC box, or in a SAP Netweaver instance. This is called embedded or Hub deployments.
Embedded Versus Hub Deployment - SAP NetWeaver Gateway Master Guide - SAP Library
Best Regards,
Emanuel