@Surya Prakash It looks like you are using the largepersongroup train API instead of persongroup train API. Does your API request URL look like the following?
https://<region>.api.cognitive.microsoft.com/face/v1.0/persongroups/{personGroupId}/train
I believe you might be using this API.
https://<region>.api.cognitive.microsoft.com/face/v1.0/largepersongroups/{largePersonGroupId}/train
Please try to use the first API and check if it works. Thanks!!