I want to fetch the task Id which will be present in the sub process.

Also I would like to suggest to go through the documentation of the function when you find time. Though you have set the context in the report, still you need to populate the context argument in the function on need basis. For instance if the report is based on user context, the function executes even without a user context as the function takes the context as the user under which the function is being executed. Whereas in case of the process model, though the context is set in the report, still the function also expects the context argument to be populated. The documentation explains this and further the same can be found in the Appian forum posts as well.

Also I would like to suggest to go through the documentation of the function when you find time. Though you have set the context in the report, still you need to populate the context argument in the function on need basis. For instance if the report is based on user context, the function executes even without a user context as the function takes the context as the user under which the function is being executed. Whereas in case of the process model, though the context is set in the report, still the function also expects the context argument to be populated. The documentation explains this and further the same can be found in the Appian forum posts as well.