Click the request /jpetstore/shop/viewCategory.shml link in the validation window to be taken automatically to the definition of the request containing the error in the scenario User Paths.
You can see that the request contains two parameters:
categoryID: This parameter provides the category of animal selected during the scenario recording (in this case a dog). The categoryId field is not generated by the server, but is a direct result of the user's choice. Consequently, it can be safely assumed that this parameter is not the source of the error.
id: The id parameter is a suspect, however, due both to its name and its value. You can assume that the value is generated by the server and that this parameter is the source of the error reported during validation.
If in doubt over a parameter, continue the process for each parameter. The next step will allow you to check whether the parameter should be processed, or whether the recorded value can be submitted unaltered.