the "Module" property of a Service task is the identifier of a custom module (e.g. "opm5") which implements the task. The module is created in the Web Project associated with the BPM Project.
The implementation of the module depends on the operations that have to be executed when you run the service task. As example, you can refer the Expense Report sample project (Help > BPM Samples). The "Process Expense" task is a Service Task and the associated module is the "Expense Report - Treasurer - Process Expense" operation module.
If you wish to associate a custom module to a service task, you have to implement the module in the Web Project (e.g. opm5), specify the "Module" property of the service task with the oid of the custom module, synchronize the BPM to Web Project. When you execute the task, the custom module should be called.
Regarding the error that you obtained: does the module property contain the oid of an existing module ? What are the executed steps to set/implement the custom module ?