Based on the error message you have provided, it is evident that the key you specified is not the unique key or in other words, does not match the key of the instance.
To see if the instance you are trying to pull from database exists or not, use pxDoesHandleExist() function rule by passing your key value. The function returns true if the instance exists in database, else false. Accordingly you can check further if the issue is with key you provided or something else.
If the function returns false, I suggest you to check the unique key for that instance in data base. Sometimes, the unique key can include other values apart from pzInsKey.