Hi Chen,
That's very strange. Just to be sure I also tried with SP 11.12.1 and had no probems. So it must have something to do with you AWS subscription. We can't help you with that.
Things to verify:
Did you subscribe to Amazon Translate and have billing in place?
Did you try to test a translation in the AWS console yourself?
And when succefull, try again with that IAM user?
Kind regards,
Emiel Paasschens
Tech Alliance Development team