S3 policy: MalformedPolicy

I am trying to apply a policy for my s3 bucket but I am getting below error… please suggest
S3 policy: MalformedPolicy

Error: Error putting S3 policy: MalformedPolicy: Missing required field Principal
status code: 400, request id: 6FTV2T57CP3Y7S9X, host id: xxxxxxx=xxxxx

on iam_fileserver_s3.tf line 1, in resource “test_s3_policy” “file_server_s3_policy”:
1: resource “aws_s3_bucket_policy” “test_s3_policy” {