Hi @Srikanth Reddy and welcome to the community,
What do you mean by "I've added a user story as subtask"? Subtasks are visible in the backlog of a CMP:
However in a TMP subtasks are not visible, but you can see a small child icon display next to the item on the backlog, signify that this story has child issues:
Welcome to the Atlassian Community!
Sub-tasks are not sprint items, they are part of their owning story. So they won't appear in the backlog (as there is no point in having them in there). They will appear in the active sprint board if their parent is in the sprint.