principal states.amazonaws.com is not authorized to assume the provided role

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I created an AWS step function using Terraform. For now, the step function has only 1 lambda function for now:

resource "aws_iam_role_policy" "sfn_policy" {
  policy = jsonencode(
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "sts:AssumeRole",
       "Resource": "*"
    },
    {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "lambda:InvokeFunction",
                "lambda:InvokeAsync"
            ],
            "Resource": "*"
    },
    {
            "Effect": "Allow",
             "Action": [ "states:StartExecution" ],
            "Resource": "*"
        }
  ]
}
  )
  role = aws_iam_role.processing_lambda_role.id
}


resource "aws_sfn_state_machine" "sfn_state_machine_zip_files" {
  name     = local.zip_files_step_function_name
  role_arn = aws_iam_role.processing_lambda_role.arn

  definition = <<EOF
{
  "Comment": "Process Incoming Zip Files",
  "StartAt": "ProcessIncomingZipFiles",
  "States": {
    "ProcessIncomingZipFiles": {
      "Type": "Task",
      "Resource": "${aws_lambda_function.process_zip_files_lambda.arn}",
      "ResultPath": "$.Output",
      "End": true
    }
  }
}
EOF
}

This is how the role is initially defined:

resource "aws_iam_role" "processing_lambda_role" {
  name = local.name
  path = "/service-role/"

  assume_role_policy = jsonencode({
    Version   = "2012-10-17"
    Statement = [
      {
        Effect    = "Allow"
        Principal = { Service = "lambda.amazonaws.com" }
        Action    = "sts:AssumeRole"
      }
    ]
  })
}

Why do I get this error message even though the policies include the AssumeRole policy already. I also tried removing one of the sts:AssumeRolepolicies but the error was still there.

"Neither the global service principal states.amazonaws.com, nor the regional one is authorized to assume the provided role."

AWS docs Reference: https://aws.amazon.com/premiumsupport/knowledge-center/step-functions-iam-role-troubleshooting/

Answer

The role aws_iam_role.processing_lambda_role can be only assumed by a lambda function. So, your aws_sfn_state_machine.sfn_state_machine_zip_files can’t assume this role. You have to change the Principal in the role from:

Principal = { Service = "lambda.amazonaws.com" }

into

Principal = { Service = "states.amazonaws.com" }

You still may have other issues, depending on what you want to do exactly. But your reported error is due to what I mentioned.