Hi Mattia,
The Process status is not directly available in Web Interface Report by default.
You have to define a measure as "Status" (with default value ex: "Not Executed") in the process that is triggered by the web interface
and update it's value in Output Variables Tab in Activity with different values("Waiting", "Approved", "Rejected","Completed").
So every time you trigger/update the process, the "Status" variable will be updated and it can be displayed in your WI report.
http://docs.runmyprocess.com/User_Guide/Development_Environment/Toolbox/Processes/Process_Measures_TabHope this could help you and feel free to ask for any additional information.
Regards,
Murali