Where/how are you executing this function? 

Since you're in offline context, my best guess is you're using 'callActivity' action. callActivity works in its own context or at best in the context of unnamed page that is created with 'data' JSON object you pass with the action. In such cases, you need to pass 'pzInsKey' of workpage from client and open corresponding work object on server to access work properties. When adding callActivity action, you should be in work context on client and write something like - 

var metadata = {
    "action": "callActivity",
    "activityName": "---",
    "className": "---",
    "parameters": [
        ---
    ]
};
var data = {
    "insKey": pega.ui.ClientCache.find("pyWorkPage").get("pzInsKey").getValue()
}

Then, in your activity you access "insKey" with myStepPage.getString("insKey") 

Or, you can pass insKey alongwith parameters in which case you can access it using Param.insKey