I’m not sure what version of the CLI the presenter is using, but it appears to not work for the version I have installed.
$ aws kms encrypt –plaintext "hello" –key-id arn:aws:kms:us-west-2:831075265234:key/62fe7757-0baa-4c6b-9fc2-778bafd1a176
Invalid base64: "hello"
$ aws –version
aws-cli/2.0.0 Python/3.7.4 Darwin/18.7.0 botocore/2.0.0dev4