I was trying in an expression rule from a processId to get the data that is inside the process variable, that is inside the process model.It is possible?
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Yes. You can write a Process Report (see: https://docs.appian.com/suite/help/21.2/Configuring_Process_Reports.html) which can get access to the process variable data. You can then invoke the Process Report from within an Expression Rule to get the data you've specified.
The steps Stewart and Harsha outlined above already cover this. What exactly is still not working for you?