Amazon Lex V2 - Test set validation error

Hi,

I generated a test set for my bot from conversation log in CloudWatch, but I got a validation error afterwards.

the error panel I got is below: There were validation errors found in this test set. Download the file to find issues in the test set file and annotate them. Then update this test set with the annotated file and you will be ready to execute a test after validation.

I can't find any validation error details and in CSV file. Does anyone know how to fix this error? What does "annotation" mean here?