IAM Policy SFTP

Reading this doc - Link: https://docs.aws.amazon.com/transfer/latest/userguide/monitoring.html

So, i have created 1 policy and attached it to new role.

When i want to create second role i have the error:
This policy contains the following error: Has prohibited field Principal For more information about the IAM policy grammar, see AWS IAM Policies

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Principal": {
"Service": "transfer.amazonaws.com"
},
"Action": "sts:AssumeRole",
"Condition": {}
}
]
}