Why RPI touchscreen does not load the Volumio interface?
Rpi touchscreen is a well supported gear and full compatible with Volumio.
If the screen does not show properly the Volumio’s user interface probably you did not install the TOUCH DISPLAY plugin, downloading it from PLUGIN section of the Volumio UI.
So, first, access to Volumio, via APP or volumio.local web browser’s address
Download the plugin
Connect RPI touchscreen
Note: RPI touchscreen and TOUCH DISPLAY are 3rd party products, please refer to their support service for issues and troubleshootings