Amazon Comprehend: Validate user metadata failed. , exit code: 255

When I try to train a Custom Entity Recognition Model using a plain text annotation file in csv format via aws console, the model training always fails with the error message:

Validate user metadata failed. , exit code: 255

I double checked the csv format and the iam role access permissions. Any clues about what's causing the issue? Don't know anywhere else to look for more details on this error