Hi @sheejo
while ingesting data to AEP _id field is required for both experience event and individual profile classes it is a unique identifier for a record which you're trying to ingest into AEP, while ingesting data sometimes you won't get an error in the data prep UI if you left/forgot to map _id field with a unique identifier make sure you create a unique record identifier to individually identify ingested records to map it to _id field in your schema.
Note: for schema based on experience event class timestamp of the event is also required.
check this for more details.