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

@ravir I would like to comment as follows, on what other practitioners has mentioned so far:

Re function:
Usage of the function entirely depends on the version. Usage of a!queryProcessAnalytics() is permitted ONLY if you are on or above Appian v7.8. Else fn!getportalreportdatasubset() (A plugin that was deprecated after the introduction of a!queryProcessAnalytics() in Appian v7.8) is an option for you.

Re approach:
You might take a look at the approach as specified above but prior to opting for it, I would like to suggest to have a look at the following:

1. You will get the task information from the sub-process (which you are aiming at) and also the main process and other sub-processes in the same. So this means that you will end up in having task information from 3 different sources, that is, the main process on which you constructed the report, the sub-process which you already have and aiming at, and the other sub-processes in the same.
So, think of applying a filter based on the sub-process from which you want the task. In simple English, the query should be like 'Get the tasks from main process which belongs to a particular sub process.'

2. In the main process model, did you configure the sub-process node (i.e. the node which is MNIed) with the option 'Allow data from this sub-process to be included in reports on the parent process model.'? If not, as per my knowledge, this approach doesn't work and you need to tweak it.

3. Did you select 'Delete previously completed/cancelled instances' on MNI'ed node? If so, as per my knowledge, this approach doesn't work definitely because the parent-child relationship will be decoupled upon selecting 'Delete previously completed/cancelled instances' and thereby they won't appear in the report.

@ravir I would like to comment as follows, on what other practitioners has mentioned so far:

Re function:
Usage of the function entirely depends on the version. Usage of a!queryProcessAnalytics() is permitted ONLY if you are on or above Appian v7.8. Else fn!getportalreportdatasubset() (A plugin that was deprecated after the introduction of a!queryProcessAnalytics() in Appian v7.8) is an option for you.

Re approach:
You might take a look at the approach as specified above but prior to opting for it, I would like to suggest to have a look at the following:

1. You will get the task information from the sub-process (which you are aiming at) and also the main process and other sub-processes in the same. So this means that you will end up in having task information from 3 different sources, that is, the main process on which you constructed the report, the sub-process which you already have and aiming at, and the other sub-processes in the same.
So, think of applying a filter based on the sub-process from which you want the task. In simple English, the query should be like 'Get the tasks from main process which belongs to a particular sub process.'

2. In the main process model, did you configure the sub-process node (i.e. the node which is MNIed) with the option 'Allow data from this sub-process to be included in reports on the parent process model.'? If not, as per my knowledge, this approach doesn't work and you need to tweak it.

3. Did you select 'Delete previously completed/cancelled instances' on MNI'ed node? If so, as per my knowledge, this approach doesn't work definitely because the parent-child relationship will be decoupled upon selecting 'Delete previously completed/cancelled instances' and thereby they won't appear in the report.