According to Preparing your training data
If you use csv file to import data, you need to assign the content to specific data set. (You need to upload your file to GCS first)
Ex.
TRAIN, gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_1}, {LABLE}
VALIDATION, gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_2}, {LABLE}
TEST, gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_3}, {LABLE}
or do not assign the data set, AutoML automatically places it in one of the three sets
gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_1}, {LABLE}
gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_2}, {LABLE}
gs://{YOUR_BUCKET_NAME}/{OBJECT_NAME_3}, {LABLE}