I have added custom field story points on a Story Ticket and its sub-tasks.
Now a Story may have multiple sub-tasks. I want to create a Rollup field that sums up the story points from all the sub-tasks and displays it on a field on the Parent Story Ticket.
How can I achieve this?
In Scrum, you set up sprints, with the aim that you commit to delivering the items you allocate into the sprint.
To help size it, you estimate your sprint items, and then use the information from previous sprints (or a flat-out guess, if you're just getting started), to try to build a manageable block of work that fits within the sprint and delivers a measurable increment on the product.
Jira's Scrum boards work off the estimate - when you complete an issue, the estimate burns down, and at the end of the sprint, what you've completed is measured against what you committed to.
So, on to the sub-tasks. You do not put sprint estimates on sub-tasks.
Scrum has nothing to say about subtasks, whether or not to use them, nor how, but, they are not sprint items, they are a part of their parent issue. If they have a sprint estimate, it's part of the parent's estimate, not their own individual estimate.
Because they are fragments, and not sprint items, they don't count towards your velocity or burn down.
If you put a sprint estimate on them, Jira simply ignores them. Which throws off all the non-sprint reporting, because that doesn't know that it should be ignoring the sub-task estimates.
If you do put the estimates on there, and accumulate them up to the parent issues, it gets worse - you might now have an accurate estimate on the parent, but it cripples your ability to do flexible estimates - you have to estimate all your sub-tasks before you can bring them into the sprint, which means you can't add sub-tasks later, your sprint reports won't reflect them as anything other than as part of their parent, and if you do it, your non-sprint reporting is now double-counting.
If you do, for some reason, want to put estimates on sprints, then you are going to need to be more clever and Scrum-aware than "just add up the estimates to the parent".